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/functionAsExpression/WithoutBody.fir.kt

@@ -9,4 +9,4 @@ fun outer() {
     bar(fun ())
     bar(l@ fun ())
     bar(@ann fun ())
-}
+}