[NI] Consider type non-nullable only if it has non-null supertype

Fixes #KT-12684 for NI

compiler/testData/diagnostics/tests/generics/nullability/considerTypeNotNullOnlyIfItHasNotNullBound.kt

@@ -0,0 +1,6 @@
+// !LANGUAGE: +NewInference
+
+inline fun <T, reified S> foo(x: T?, y: T): T {
+    if (x is S) return <!TYPE_MISMATCH, TYPE_MISMATCH!>x<!>
+    return y
+}

compiler/testData/diagnostics/tests/generics/nullability/considerTypeNotNullOnlyIfItHasNotNullBound.txt

@@ -0,0 +1,3 @@
+package
+
+public inline fun </*0*/ T, /*1*/ reified S> foo(/*0*/ x: T?, /*1*/ y: T): T