[NI] Make return type of !! operator definitely not-null

compiler/testData/codegen/box/regressions/nullableAfterExclExcl.kt

@@ -0,0 +1,13 @@
+interface Sample {
+    val callMe: Int
+}
+
+class Caller<out M : Sample?>(val member: M) {
+    fun test() {
+        member!!.callMe
+    }
+}
+
+fun box(): String {
+    return "OK"
+}