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/testsWithStdLib/contracts/controlflow/flowInlining/labeledReturns.fir.kt

@@ -83,4 +83,4 @@ fun threeLevelsReturnWithUnknown(x: Int?): Int? {
         }
     }
     return <!UNINITIALIZED_VARIABLE!>y<!>.inc()
-}
+}