cmd/compile: use fuse to implement shortcircuit loop

cmd/compile: use fuse to implement shortcircuit loop

The rewrite loop in shortcircuit is identical to the one in fuse.
That's not surprising; shortcircuit is fuse-like.

Take advantage of that by merging the two loops.

Passes toolstash-check.

Change-Id: I642cb39a23d2ac8964ed577678f062fce721439c
Reviewed-on: https://go-review.googlesource.com/c/go/+/229003
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>