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/notAFunctionLabel_after.fir.kt

@@ -51,12 +51,12 @@ fun testLoopLabelInReturn(xs: List<Int>) {
 }
 
 fun testValLabelInReturn() {
-    L@ val fn = { <!NOT_A_FUNCTION_LABEL!>return@L<!> }
+    <!REDUNDANT_LABEL_WARNING!>L@<!> val fn = { <!NOT_A_FUNCTION_LABEL!>return@L<!> }
     fn()
 }
 
 fun testHighOrderFunctionCallLabelInReturn() {
-    L@ run {
+    <!REDUNDANT_LABEL_WARNING!>L@<!> run {
         <!NOT_A_FUNCTION_LABEL!>return@L<!>
     }
 }