Prohibit check for instance of a non-reified, non-subtype type parameter

#KT-12683 Fixed
 #KT-9986 Fixed

compiler/testData/diagnostics/testsWithStdLib/coroutines/inference/kt15516.kt

@@ -1,6 +1,6 @@
 // SKIP_TXT
 class StateMachine<Q> internal constructor() {
-    fun getInputStub(): Q = null as Q
+    fun getInputStub(): Q = <!UNCHECKED_CAST!>null as Q<!>
 }
 
 fun <T> stateMachine(<!UNUSED_PARAMETER!>block<!>: suspend StateMachine<T>.() -> Unit): StateMachine<T> {
@@ -8,7 +8,7 @@ fun <T> stateMachine(<!UNUSED_PARAMETER!>block<!>: suspend StateMachine<T>.() ->
 }
 
 class Problem<F>(){
-    fun getInputStub(): F = null as F
+    fun getInputStub(): F = <!UNCHECKED_CAST!>null as F<!>
 
     fun createStateMachine(): StateMachine<F> = stateMachine {
         val letter = getInputStub()