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/controlFlowAnalysis/deadCode/commentsInDeadCode.fir.kt

@@ -12,7 +12,7 @@ fun test2() {
     <!UNREACHABLE_CODE!>bar(<!>11, todo()/*comment1*/, <!UNREACHABLE_CODE!>""/*comment2*/)<!>
 }
 fun test3() {
-    <!UNREACHABLE_CODE!>bar(<!>11, <!UNREACHABLE_CODE!>l@(<!>todo()/*comment*/<!UNREACHABLE_CODE!>), "")<!>
+    <!UNREACHABLE_CODE!>bar(<!>11, <!UNREACHABLE_CODE!><!REDUNDANT_LABEL_WARNING!>l@<!>(<!>todo()/*comment*/<!UNREACHABLE_CODE!>), "")<!>
 }
 
 fun todo(): Nothing = throw Exception()