cmd/compile: add ellipsis syntax for op-only rewrite rules

cmd/compile: add ellipsis syntax for op-only rewrite rules

This change introduces a new syntax for rewrite rules
that only change a Value's Op. See #36380 for more discussion.

Updating rewrite rules to use ellipses will happen
in follow-up CLs.

Change-Id: I8c56e85de24607579d79729575c89ca80805ba5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/213898
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>