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/tests-spec/testData/diagnostics/notLinked/contracts/declarations/contractBuilder/common/neg/14.fir.kt

@@ -31,7 +31,7 @@ inline fun case_4(block: () -> Unit) {
 
 // TESTCASE NUMBER: 5
 inline fun case_5(block: () -> Unit) {
-    test@ contract {
+    <!REDUNDANT_LABEL_WARNING!>test@<!> contract {
         callsInPlace(block, InvocationKind.EXACTLY_ONCE)
     }
     return block()