Prefer stable types to diagnose useless cast

#KT-11622 Fixed

compiler/testData/diagnostics/tests/Casts.kt

@@ -8,11 +8,11 @@ fun test() : Unit {
   checkSubtype<Int>(y)
   checkSubtype<Int>(x as Int)
   checkSubtype<Int>(y <!USELESS_CAST!>as Int<!>)
-  checkSubtype<Int?>(x <!USELESS_CAST!>as Int?<!>)
+  checkSubtype<Int?>(x as Int?)
   checkSubtype<Int?>(y as Int?)
   checkSubtype<Int?>(x <!USELESS_CAST!>as? Int<!>)
   checkSubtype<Int?>(y <!USELESS_CAST!>as? Int<!>)
-  checkSubtype<Int?>(x <!USELESS_CAST!>as? Int?<!>)
+  checkSubtype<Int?>(x as? Int?)
   checkSubtype<Int?>(y as? Int?)
 
   val <!UNUSED_VARIABLE!>s<!> = "" as Any

compiler/testData/diagnostics/tests/cast/StableTypeForUselessCast.kt

@@ -0,0 +1,14 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+interface I1
+interface I2
+
+fun foo(i: I1) {}
+fun foo(i: I2) {}
+
+fun bar(i: I1) {
+    if (i is I2) {
+        foo(i as I1)
+        foo(i as I2)
+    }
+}

compiler/testData/diagnostics/tests/cast/StableTypeForUselessCast.txt

@@ -0,0 +1,17 @@
+package
+
+public fun bar(/*0*/ i: I1): kotlin.Unit
+public fun foo(/*0*/ i: I1): kotlin.Unit
+public fun foo(/*0*/ i: I2): kotlin.Unit
+
+public interface I1 {
+    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
+}
+
+public interface I2 {
+    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
+}