[dev.regabi] cmd/compile: introduce OMETHEXPR instead of overloading ONAME
A method expression today is an ONAME that has none of the
invariants or properties of other ONAMEs and is always a special case
(hence the Node.IsMethodExpression method).
Remove the special cases by making a separate Op.
Passes toolstash -cmp.
Change-Id: I7667693c9155d5486a6924dbf75ebb59891c4afc
Reviewed-on: https://go-review.googlesource.com/c/go/+/272867
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>