[JS IR] Optimize away upcasts

#KT-50212 Fixed

compiler/testData/codegen/boxError/semantic/missedBody.kt

@@ -4,9 +4,9 @@
 // MODULE: lib
 // FILE: t.kt
 
-fun bar(a: String, b: String): String
+fun bar(a: String, b: String): Any
 
-fun foo(): String {
+fun foo(): Any {
     return bar("O", "K")
 }
 
@@ -17,4 +17,4 @@ fun box(): String {
     val r = foo()
     if (r is String) return r
     return "OK"
-}
+}