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/controlFlowAnalysis/referenceToPropertyInitializer.fir.kt

@@ -19,7 +19,7 @@ class TestObjectLiteral {
             val y = obj
         }
     }
-    val obj1: A = l@ ( object: A(obj1) {
+    val obj1: A = <!REDUNDANT_LABEL_WARNING!>l@<!> ( object: A(obj1) {
         init {
             val x = obj1
         }