FIR checker: support DUPLICATE_LABEL_IN_WHEN

Changes from FE1.0:
1. As discussed previously, no expression evaluation happens during this
check.
2. FE1.0 doesn't check redundant object comparisons.

compiler/testData/diagnostics/tests/when/DuplicatedLabels.kt

@@ -41,6 +41,7 @@ fun third(arg: Any?): Int {
         is Double -> return 1
         is <!DUPLICATE_LABEL_IN_WHEN!>Double<!> -> return 2
         <!DUPLICATE_LABEL_IN_WHEN!>null<!> -> return 3
+        !is String -> return 4
         else -> return 5
     }
 }
@@ -59,3 +60,11 @@ fun fifth(arg: Any?) = when (arg) {
     <!ELSE_MISPLACED_IN_WHEN!>else<!> -> ""
     <!UNREACHABLE_CODE!>else -> null<!>
 }
+
+object Foo
+
+fun sixth(arg: Any?) = when (arg) {
+    Foo -> ""
+    Foo -> ""
+    else -> null
+}