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/BreakContinue.fir.kt

@@ -2,7 +2,7 @@ class C {
 
     fun f (a : Boolean, b : Boolean) {
         b@ while (true)
-          a@ {
+          <!REDUNDANT_LABEL_WARNING!>a@<!> {
             <!NOT_A_LOOP_LABEL!>break@f<!>
             break
             break@b
@@ -12,7 +12,7 @@ class C {
         <!BREAK_OR_CONTINUE_OUTSIDE_A_LOOP!>continue<!>
 
         b@ while (true)
-          a@ {
+          <!REDUNDANT_LABEL_WARNING!>a@<!> {
             <!NOT_A_LOOP_LABEL!>continue@f<!>
             continue
             continue@b