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/controlStructures/forWithNullableIterator.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class Coll {
   operator fun iterator(): It? = null
 }