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/controlStructures/redundantLabel.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 @Target(AnnotationTarget.EXPRESSION)
 @Retention(AnnotationRetention.SOURCE)
 annotation class Ann