[dev.typeparams] cmd/compile: add method value wrappers to unified IR

[dev.typeparams] cmd/compile: add method value wrappers to unified IR

Method value wrappers will need dictionary support too, so bring them
under the unified IR umbrella as well.

Change-Id: Iec36bb04efdf59843d1b00f55d2c44bc841fa2ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/331190
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>