FIR DFA: move eq/notEq null-to-type translation to LogicSystem

This makes the `returns() implies` checker slightly cleaner, and also
fixes the case that I've missed where in RHS of `x ?:` type of `x` was
not set to `Nothing?`.

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/14.fir.kt

@@ -19,7 +19,7 @@ fun case_2(x: Int?) {
 
 // TESTCASE NUMBER: 3
 fun case_3(x: Boolean?) {
-    if (x ?: (x != null)) {
+    if (x ?: (<!SENSELESS_COMPARISON!>x != null<!>)) {
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!>
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!><!UNSAFE_CALL!>.<!>not()
     }

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/39.fir.kt

@@ -11,8 +11,8 @@ fun case_1() {
     while (true) {
         println(x ?: break)
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String?")!>x<!><!UNSAFE_CALL!>.<!>length
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.String? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>length
 }
 
 /*
@@ -25,8 +25,8 @@ fun case_2(y: MutableList<Int>) {
     while (true) {
         y[x ?: break] = 10
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 /*
@@ -39,8 +39,8 @@ fun case_3(y: MutableList<Int>) {
     while (true) {
         y[0] = x ?: break
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 // TESTCASE NUMBER: 4
@@ -49,8 +49,8 @@ fun case_4() {
     while (true) {
         x ?: break
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 // TESTCASE NUMBER: 5
@@ -59,8 +59,8 @@ fun case_5(y: Boolean) {
     while (true) {
         y && (x ?: break)
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!><!UNSAFE_CALL!>.<!>not()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>not()
 }
 
 // TESTCASE NUMBER: 6
@@ -69,8 +69,8 @@ fun case_6(y: Boolean) {
     while (true) {
         y || (x ?: break)
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!><!UNSAFE_CALL!>.<!>not()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>not()
 }
 
 // TESTCASE NUMBER: 7
@@ -94,8 +94,8 @@ fun case_8() {
     while (true) {
         y += x ?: break
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 /*
@@ -109,8 +109,8 @@ fun case_9() {
     while (true) {
         y -= x ?: break
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 /*
@@ -124,8 +124,8 @@ fun case_10() {
     while (true) {
         val z = y - (x ?: break)
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 /*
@@ -139,8 +139,8 @@ fun case_11() {
     while (true) {
         val z = y * (x ?: break)
     }
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!>
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x<!><!UNSAFE_CALL!>.<!>inv()
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int? & kotlin.Nothing?")!>x<!><!UNSAFE_CALL!>.<!>inv()
 }
 
 // TESTCASE NUMBER: 12

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/7.fir.kt

@@ -19,7 +19,7 @@ fun case_2(x: Int?) {
 
 // TESTCASE NUMBER: 3
 fun case_3(x: Boolean?) {
-    if (x ?: (x != null)) {
+    if (x ?: (<!SENSELESS_COMPARISON!>x != null<!>)) {
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!>
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Boolean?")!>x<!><!UNSAFE_CALL!>.<!>not()
     }

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/35.fir.kt

@@ -48,7 +48,7 @@ fun case_4(x: Any?) {
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Any")!>x<!>.equals(10)
     }
 
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any?")!>x<!>
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & kotlin.Nothing?")!>x<!>
 }
 
 // TESTCASE NUMBER: 5