[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/controlFlowAnalysis/unresolvedReference.kt

@@ -1,4 +1,3 @@
-// !WITH_NEW_INFERENCE
 // See KT-6665: unresolved reference (v.bar) should not produce "unreachable code" after it
 
 fun foo(): Int {
@@ -29,14 +28,14 @@ fun foo3(): Int {
 fun bar(): Int {
     val v = 1
     val c = v.<!UNRESOLVED_REFERENCE!>bar<!> ?: 42
-    return <!NI;DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>c<!>
+    return c
 }
 
 fun bar2(): Int {
     val v = 1
     val c = if (true) v.<!UNRESOLVED_REFERENCE!>bar<!> else 3
-    val b = <!NI;DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>c<!>
-    return <!NI;DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>b<!>
+    val b = c
+    return b
 }
 
 fun bar3(): Int {
@@ -45,6 +44,6 @@ fun bar3(): Int {
         true -> v.<!UNRESOLVED_REFERENCE!>bar<!>
         else -> 3
     }
-    val b = <!NI;DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>c<!>
-    return <!NI;DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>b<!>
+    val b = c
+    return b
 }