Smart casts to nothing after comparison with null

compiler/testData/diagnostics/tests/smartCasts/notNullorNotNull.kt

@@ -0,0 +1,6 @@
+fun bar(x: Int?): Int {
+    if (x != null) return -1
+    if (<!SENSELESS_COMPARISON!>x == null<!>) return -2
+    // Should be unreachable
+    return 2 + 2
+}

compiler/testData/diagnostics/tests/smartCasts/notNullorNotNull.txt

@@ -0,0 +1,3 @@
+package
+
+public fun bar(/*0*/ x: kotlin.Int?): kotlin.Int