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()
     }