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/Casts.fir.kt

@@ -1,23 +0,0 @@
-// !CHECK_TYPE
-
-fun test() : Unit {
-  var x : Int? = 0
-  var y : Int = 0
-
-  checkSubtype<Int?>(x)
-  checkSubtype<Int>(y)
-  checkSubtype<Int>(x as Int)
-  checkSubtype<Int>(y <!USELESS_CAST!>as Int<!>)
-  checkSubtype<Int?>(x as Int?)
-  checkSubtype<Int?>(y as Int?)
-  checkSubtype<Int?>(x as? Int)
-  checkSubtype<Int?>(y as? Int)
-  checkSubtype<Int?>(x as? Int?)
-  checkSubtype<Int?>(y as? Int?)
-
-  val s = "" as Any
-  ("" as String?)?.length
-  (data@("" as String?))?.length
-  (<!WRONG_ANNOTATION_TARGET!>@MustBeDocumented()<!>( "" as String?))?.length
-  Unit
-}