Do not remove trivial substitutions

to avoid substituting a new type parameter by default instead of leaving the same
 #KT-4887 Fixed

compiler/testData/diagnostics/tests/substitutions/kt4887.kt

@@ -0,0 +1,8 @@
+package h
+
+public class MyClass<S, T>(param: MyClass<S, T>) {
+    fun test() {
+        val result: MyClass<Any, Any>? = null
+        MyClass<S, Any>(result <!CAST_NEVER_SUCCEEDS!>as<!> MyClass<S, Any>)
+    }
+}