FIR checker: report REDUNDANT_LABEL_WARNING

Since many labels are not present in the FIR tree, this checker is
implemented as a syntax checker. Comparing with FE1.0, this change
reports some REDUNDANT_LABEL_WARNING that FE1.0 has missed, especially
LHS of assignments.

compiler/testData/diagnostics/tests/deparenthesize/labeledSafeCall.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun f(s : String?) : Boolean {
     return <!REDUNDANT_LABEL_WARNING!>foo@<!>(s?.equals("a"))!!
 }