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/nullabilityAndSmartCasts/QualifiedExpressionNullability.fir.kt

@@ -32,7 +32,7 @@ fun main() {
     val w: Foo? = null
     w<!UNSAFE_CALL!>.<!>f = z
     (w<!UNSAFE_CALL!>.<!>f) = z
-    (label@ w<!UNSAFE_CALL!>.<!>f) = z
+    (<!REDUNDANT_LABEL_WARNING!>label@<!> w<!UNSAFE_CALL!>.<!>f) = z
     w!!.f = z
     w.f = z
     w<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.f = z