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/FunctionCalleeExpressions.fir.kt

@@ -62,7 +62,7 @@ fun main1() {
     {1}();
     (fun (x : Int) = x)(1)
     1.(<!FUNCTION_EXPECTED!>fun Int.(x : Int) = x<!>)(1);
-    l@{1}()
+    <!REDUNDANT_LABEL_WARNING!>l@<!>{1}()
     1.((<!FUNCTION_EXPECTED!>fun Int.() = 1<!>))()
     1.(<!FUNCTION_EXPECTED!>f()<!>)()
     1.<!FUNCTION_EXPECTED!>if(true){f()}else{f()}<!>()