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/delegatedProperty/inference/labeledDelegatedExpression.fir.kt

@@ -1,11 +0,0 @@
-import kotlin.reflect.KProperty
-
-class A3 {
-    val a: String by l@ MyProperty()
-
-    class MyProperty<T> {}
-
-    operator fun <T> MyProperty<T>.getValue(thisRef: Any?, desc: KProperty<*>): T {
-        throw Exception("$thisRef $desc")
-    }
-}

compiler/testData/diagnostics/tests/delegatedProperty/inference/labeledDelegatedExpression.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 import kotlin.reflect.KProperty
 
 class A3 {