FirForLoopChecker: report also OPERATOR_MODIFIER if appropriate + minor

This commits checks iterator/hasNext/next functions whether they are
declared as operator or not. Also, it changes logic of hasNext/next
error reporting, now we're able to report errors about both these
functions.

compiler/testData/diagnostics/tests/inference/reportingImprovements/typeInferenceFailedOnIteratorCall.fir.kt

@@ -4,6 +4,6 @@ class X
 operator fun <T> X.iterator(): Iterable<T> = TODO()
 
 fun test() {
-    for (i in <!HAS_NEXT_MISSING!>X()<!>) {
+    for (i in <!HAS_NEXT_MISSING, NEXT_MISSING!>X()<!>) {
     }
 }