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/deparenthesize/ParenthesizedVariable.fir.kt

@@ -1,3 +1,3 @@
 fun test() {
-    (d@ val bar = 2)
-}
+    (<!REDUNDANT_LABEL_WARNING!>d@<!> val bar = 2)
+}

compiler/testData/diagnostics/tests/deparenthesize/checkDeparenthesizedType.fir.kt

@@ -6,19 +6,19 @@ import checkSubtype
 
 fun test(i: Int?) {
     if (i != null) {
-        foo(l1@ i)
+        foo(<!REDUNDANT_LABEL_WARNING!>l1@<!> i)
         foo((i))
-        foo(l2@ (i))
-        foo((l3@ i))
+        foo(<!REDUNDANT_LABEL_WARNING!>l2@<!> (i))
+        foo((<!REDUNDANT_LABEL_WARNING!>l3@<!> i))
     }
 
-    val a: Int = <!INITIALIZER_TYPE_MISMATCH!>l4@ ""<!>
+    val a: Int = <!INITIALIZER_TYPE_MISMATCH!><!REDUNDANT_LABEL_WARNING!>l4@<!> ""<!>
     val b: Int = <!INITIALIZER_TYPE_MISMATCH!>("")<!>
     val c: Int = checkSubtype<Int>(<!ARGUMENT_TYPE_MISMATCH!>""<!>)
     val d: Int = <!INITIALIZER_TYPE_MISMATCH!>checkSubtype<Long>(<!ARGUMENT_TYPE_MISMATCH!>""<!>)<!>
 
 
-    foo(l4@ <!ARGUMENT_TYPE_MISMATCH!>""<!>)
+    foo(<!REDUNDANT_LABEL_WARNING!>l4@<!> <!ARGUMENT_TYPE_MISMATCH!>""<!>)
     foo((<!ARGUMENT_TYPE_MISMATCH!>""<!>))
     foo(checkSubtype<Int>(<!ARGUMENT_TYPE_MISMATCH!>""<!>))
     foo(<!ARGUMENT_TYPE_MISMATCH!>checkSubtype<Long>(<!ARGUMENT_TYPE_MISMATCH!>""<!>)<!>)

compiler/testData/diagnostics/tests/deparenthesize/labeledSafeCall.fir.kt

@@ -1,3 +0,0 @@
-fun f(s : String?) : Boolean {
-    return foo@(s?.equals("a"))!!
-}

compiler/testData/diagnostics/tests/deparenthesize/labeledSafeCall.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun f(s : String?) : Boolean {
     return <!REDUNDANT_LABEL_WARNING!>foo@<!>(s?.equals("a"))!!
 }