[NI] Make subtyping algorithm more robust to error types

During subtyping/incorporation we transform types (e.g. changing nullability,
 form of the type) and, basically, we're doing this to some FIXPOINT.
 It's important that we use `KotlinType.hashCode()` to compare types, but
 for error types hashCode is a hashCode of its supertype and, for example,
 `makeNullableAsSpecified` method recreate type every time. So, we continue
 to generate new constraints and we'll never stop incorporation algorithm

compiler/testData/diagnostics/tests/functionLiterals/return/ForbiddenNonLocalReturnNoType.kt

@@ -1,18 +1,18 @@
 // !WITH_NEW_INFERENCE
 fun test() {
     run {<!RETURN_NOT_ALLOWED!>return<!>}
-    run {}
+    <!NI;UNREACHABLE_CODE!>run {}<!>
 }
 
 fun test2() {
     run {<!RETURN_NOT_ALLOWED!>return@test2<!>}
-    run {}
+    <!NI;UNREACHABLE_CODE!>run {}<!>
 }
 
 fun test3() {
     fun test4() {
         run {<!RETURN_NOT_ALLOWED!>return@test3<!>}
-        run {}
+        <!NI;UNREACHABLE_CODE!>run {}<!>
     }
 }