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/ReserveYield2.fir.kt

@@ -5,8 +5,8 @@
 annotation class yield
 
 fun bar(p: Int) {
-    yield@ p
-    `yield`@ p
+    <!REDUNDANT_LABEL_WARNING!>yield@<!> p
+    <!REDUNDANT_LABEL_WARNING!>`yield`@<!> p
 
     @yield() p
     @`yield`() p