Make InsertImplicitCasts work with Descriptors and KotlinTypes again

This is a temporary change to make sure IrType infrastructure work is in
master.

It causes some of irText tests to work incorrectly (as in master).

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/TypeOperatorLowering.kt

@@ -124,6 +124,7 @@ class TypeOperatorLowering(val context: JsIrBackendContext) : FileLoweringPass {
             }
 
             private fun lowerImplicitDynamicCast(expression: IrTypeOperatorCall) = expression.run {
+                // TODO check argument
                 assert(operator == IrTypeOperator.IMPLICIT_DYNAMIC_CAST)
                 argument
             }