[FIR] Add INAPPLICABLE_OPERATOR_MODIFIER diagnostic

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

@@ -1,10 +0,0 @@
-class IncDec {
-    operator fun inc(): Unit {}
-}
-
-fun foo(): IncDec {
-    var x = IncDec()
-    x = x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-    x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-    return x
-}

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class IncDec {
     <!INAPPLICABLE_OPERATOR_MODIFIER!>operator<!> fun inc(): Unit {}
 }