cmd/compile: CSE the RHS of rewrite rules

cmd/compile: CSE the RHS of rewrite rules

Keep track of all expressions encountered while
generating a rewrite result, and re-use them whenever possible.
Named expressions may still be used for clarity when desired.

Change-Id: I640dca108763eb8baeff8f9a4169300af3445b82
Reviewed-on: https://go-review.googlesource.com/c/go/+/229800
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>