Check separately auto casts for nullable types.

It's necessary for functions with generics: constraint system ignores nullability divergence,
the error checks are made afterwards, the auto cast check should be made similarly.

compiler/testData/diagnostics/tests/nullabilityAndAutoCasts/smartCastedReceiverWithGenerics.kt

@@ -0,0 +1,7 @@
+fun test(a: Any?) {
+    if (a != null) {
+        <!DEBUG_INFO_AUTOCAST!>a<!>.foo(11)
+    }
+}
+
+fun <T> Any.foo(t: T) = t