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/controlStructures/redundantLabel.fir.kt

@@ -1,39 +0,0 @@
-@Target(AnnotationTarget.EXPRESSION)
-@Retention(AnnotationRetention.SOURCE)
-annotation class Ann
-
-fun testLambdaLabel() = l@ { 42 }
-
-fun testAnonymousFunctionLabel() = l@ fun() {}
-
-fun testAnnotatedLambdaLabel() = lambda@ @Ann {}
-
-fun testParenthesizedLambdaLabel() = lambda@ ( {} )
-
-fun testLabelBoundToInvokeOperatorExpression() = l@ { 42 }()
-
-fun testLabelBoundToLambda() = (l@ { 42 })()
-
-fun testWhileLoopLabel() {
-    L@ while (true) {}
-}
-
-fun testDoWhileLoopLabel() {
-    L@ do {} while (true)
-}
-
-fun testForLoopLabel(xs: List<Any>) {
-    L@ for (x in xs) {}
-}
-
-fun testValLabel() {
-    L@ val fn = {}
-    fn()
-}
-
-fun testHighOrderFunctionCallLabel() {
-    L@ run {}
-}
-
-fun testAnonymousObjectLabel() =
-    L@ object {}