[dev.typeparams] cmd/compile: port CL 330838 for -G=3

So next CL can move MethodValueWrapper to walk, since when walk is now
the only place which uses this function.

Change-Id: Id3be359bfc38efb022451cb7f9e53c2868fe7e12
Reviewed-on: https://go-review.googlesource.com/c/go/+/330840


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>