cmd/compile: leave the horses alone

cmd/compile: leave the horses alone

I don't know why we were eliminating horses, this not cool.

Change-Id: I0d4b5a1b2f584e071de0a85ef88f9baf9183e12e
Reviewed-on: https://go-review.googlesource.com/c/go/+/712820
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>