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/PackageInExpressionPosition.fir.kt

@@ -22,6 +22,6 @@ fun main() {
     <!NO_COMPANION_OBJECT!>System<!> is Int
     <!INVISIBLE_REFERENCE!>System<!>()
     (<!NO_COMPANION_OBJECT!>System<!>)
-    foo@ <!NO_COMPANION_OBJECT!>System<!>
+    <!REDUNDANT_LABEL_WARNING!>foo@<!> <!NO_COMPANION_OBJECT!>System<!>
     null <!UNRESOLVED_REFERENCE!>in<!> System
 }