Fix overload ambiguity after smartcast to nullable Nothing

#KT-39544 Fixed

compiler/testData/diagnostics/tests/smartCasts/smartcastToNothingAfterCheckingForNull.fir.kt

@@ -0,0 +1,29 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+// FILE: B.java
+
+public abstract class B<T> implements A<T> {}
+
+// FILE: test.kt
+
+interface A<T> {
+    val content: T
+}
+fun f(x: Any?) {}
+fun f(x: Byte) {}
+fun f(x: Char) {}
+
+fun g(i: Int) {}
+
+fun g(x: B<Int>) {
+    val y = x.content
+    if (y == null) {
+        f(y)
+        g(y)
+    }
+
+    if (y is Nothing?) {
+        <!AMBIGUITY!>f<!>(y)
+        <!AMBIGUITY!>g<!>(y)
+    }
+}