FIR checker: warn unnecessary non-null assertions

compiler/testData/diagnostics/tests/smartCasts/safecalls/argument.fir.kt

@@ -1,6 +0,0 @@
-fun calc(x: List<String>?): Int {
-    // After KT-5840 fix !! assertion should become unnecessary here
-    x?.get(x!!.size - 1)
-    // x?. or x!! above should not provide smart cast here
-    return x<!UNSAFE_CALL!>.<!>size
-}

compiler/testData/diagnostics/tests/smartCasts/safecalls/argument.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun calc(x: List<String>?): Int {
     // After KT-5840 fix !! assertion should become unnecessary here
     x?.get(x<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.size - 1)

compiler/testData/diagnostics/tests/smartCasts/safecalls/propertyChain.fir.kt

@@ -10,5 +10,5 @@ fun foo(y: MyClass?): Int {
 fun bar(y: MyClass?) {
     y?.x!!.length
     // !! is NOT necessary here, because y?.x != null
-    y!!.x
-}
+    y<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.x
+}