FIR DFA: smartcast variable to Nothing? on null assignment

In order to make resolution still work for members not available from
`Nothing`, we track the type without `Nothing?` and use that for
resolution instead.

compiler/testData/diagnostics/tests/smartCasts/alwaysNull.fir.kt

@@ -7,7 +7,7 @@ fun foo(): String {
     var t: String? = "y"
     if (t == null) t = "x"
     var x: Int? = null
-    if (x == null) x <!UNSAFE_OPERATOR_CALL!>+=<!> null
+    if (x == null) <!ASSIGNMENT_TYPE_MISMATCH!>x += null<!>
     return t + s
 }
 
@@ -16,7 +16,7 @@ fun String?.gav() {}
 fun bar(s: String?) {
     if (s != null) return
     s.gav()
-    s <!USELESS_CAST!>as? String<!>
-    s <!USELESS_CAST!>as String?<!>
+    s as? String
+    s as String?
     s as String
 }

compiler/testData/diagnostics/tests/smartCasts/alwaysNullWithJava.fir.kt

@@ -1,16 +0,0 @@
-// FILE: My.java
-
-public class My {
-    static public My create() { return new My(); }
-    public void foo() {}
-}
-
-// FILE: Test.kt
-
-fun test() {
-    val my = My.create()
-    if (my == null) {
-        my.foo()
-    }
-}
-

compiler/testData/diagnostics/tests/smartCasts/alwaysNullWithJava.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // FILE: My.java
 
 public class My {

compiler/testData/diagnostics/tests/smartCasts/equals.fir.kt

@@ -7,14 +7,14 @@ class TestWithEquals {
 }
 
 fun bar(i: Test?) {
-    if (i == null) foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
+    if (i == null) foo(i)
 }
 
 fun bar(i: TestWithEquals?) {
-    if (i == null) foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
-    if (null == i) foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
+    if (i == null) foo(i)
+    if (null == i) foo(i)
     when (i) {
-        null -> foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
+        null -> foo(i)
     }
 }
 
@@ -22,4 +22,4 @@ fun gav(i: TestWithEquals?, j: TestWithEquals?) {
     if (j == null) {
         if (i == j) foo(<!ARGUMENT_TYPE_MISMATCH!>i<!>)
     }
-}
+}

compiler/testData/diagnostics/tests/smartCasts/smartcastToNothingAfterCheckingForNull.fir.kt

@@ -19,11 +19,11 @@ fun g(x: B<Int>) {
     val y = x.content
     if (y == null) {
         f(y)
-        g(y)
+        <!NONE_APPLICABLE!>g<!>(y)
     }
 
     if (y is Nothing?) {
         f(y)
-        g(y)
+        <!NONE_APPLICABLE!>g<!>(y)
     }
 }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/varIntNull.fir.kt

@@ -1,5 +1,5 @@
 fun foo(): Int {
     var i: Int? = 42
     i = null
-    return i <!UNSAFE_OPERATOR_CALL!>+<!> 1
+    return <!RETURN_TYPE_MISMATCH, TYPE_MISMATCH!>i + 1<!>
 }