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
It helps to catch TYPECHECKER_HAS_RUN_INTO_RECURSIVE_PROBLEM errors
because they appear after return type calculation began
#KT-6271 Fixed
#KT-3272 Obsolete
Dmitriy Novozhilov