cmd/compile: refactoring mixing untyped type logic

defaultlit2 and typecheck use the same logic for getting mixing untyped
type, so move that logic to a function.

This is a followup of CL 255217.

Passes toolstash-check.

Change-Id: Ic0eadb7ed27a2f0f72e2d28fd5438500bf4c79e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/255897


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>