[dev.typeparams] cmd/compile: rename OCALLPART to OMETHVALUE

Go spec call them "method values", not "partial calls". Note that
we use "OMETHVALUE" (as opposed to "OMETHODVALUE") to be consistent
with "OMETHEXPR".

Change-Id: I1efd985d4b567a1b4b20aeb603eb82db579edbd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/330837


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>