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.fir.kt

@@ -15,10 +15,10 @@ const val four = 4
 fun first(arg: Int) = when (arg) {
     1 -> 2
     2 -> 3
-    1 -> 4
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 4
     4 -> 5
-    1 -> 6
-    2 -> 7
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 6
+    <!DUPLICATE_LABEL_IN_WHEN!>2<!> -> 7
     // Error should be here: see KT-11971
     four -> 8
     else -> 0
@@ -28,8 +28,8 @@ fun second(arg: String): Int {
     when (arg) {
         "ABC" -> return 0
         "DEF" -> return 1
-        "ABC" -> return -1
-        "DEF" -> return -2
+        <!DUPLICATE_LABEL_IN_WHEN!>"ABC"<!> -> return -1
+        <!DUPLICATE_LABEL_IN_WHEN!>"DEF"<!> -> return -2
     }
     return 42
 }
@@ -39,8 +39,9 @@ fun third(arg: Any?): Int {
         null -> return -1
         is String -> return 0
         is Double -> return 1
-        is Double -> return 2
-        null -> return 3
+        is <!DUPLICATE_LABEL_IN_WHEN!>Double<!> -> return 2
+        <!DUPLICATE_LABEL_IN_WHEN!>null<!> -> return 3
+        !is String -> return 4
         else -> return 5
     }
 }
@@ -50,7 +51,7 @@ enum class Color { RED, GREEN, BLUE }
 fun fourth(arg: Color) = when (arg) {
     Color.RED -> "RED"
     Color.GREEN -> "GREEN"
-    Color.RED -> "BLUE"
+    <!DUPLICATE_LABEL_IN_WHEN!>Color.RED<!> -> "BLUE"
     Color.BLUE -> "BLUE"
 }
 
@@ -59,3 +60,11 @@ fun fifth(arg: Any?) = when (arg) {
     <!ELSE_MISPLACED_IN_WHEN!>else<!> -> ""
     else -> null
 }
+
+object Foo
+
+fun sixth(arg: Any?) = when (arg) {
+    Foo -> ""
+    <!DUPLICATE_LABEL_IN_WHEN!>Foo<!> -> ""
+    else -> null
+}

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
+}

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

@@ -6,6 +6,7 @@ package test {
     public fun first(/*0*/ arg: kotlin.Int): kotlin.Int
     public fun fourth(/*0*/ arg: test.Color): kotlin.String
     public fun second(/*0*/ arg: kotlin.String): kotlin.Int
+    public fun sixth(/*0*/ arg: kotlin.Any?): kotlin.String?
     public fun third(/*0*/ arg: kotlin.Any?): kotlin.Int
 
     public final enum class Color : kotlin.Enum<test.Color> {
@@ -30,4 +31,12 @@ package test {
         public final /*synthesized*/ fun valueOf(/*0*/ value: kotlin.String): test.Color
         public final /*synthesized*/ fun values(): kotlin.Array<test.Color>
     }
+
+    public object Foo {
+        private constructor Foo()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
 }
+