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/functionLiterals/return/IfWithoutElse.fir.kt

@@ -12,4 +12,4 @@ val b/*: () -> Int */ = l@ {
 
 val c/*: () -> Unit */ = l@ {
     if (flag) 4
-}
+}