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/ParenthesizedVariable.fir.kt

@@ -1,3 +1,3 @@
 fun test() {
-    (d@ val bar = 2)
-}
+    (<!REDUNDANT_LABEL_WARNING!>d@<!> val bar = 2)
+}