cmd/compile: reduce rulelog noise

cmd/compile: reduce rulelog noise

When the SSA rules are re-generated to log rules,
they write output like:

rewrite AMD64.rules:527
rewrite AMD64.rules:427
rewrite AMD64.rules:494

This is silly; there are no non-rewrite lines in the file.
Furthermore, the rulelog file tends to be gigantic
for any non-trivial compilation (measured in gigabytes).

Remove the "rewrite " prefix.

No impact to normal builds.

Change-Id: I955995c1cc5f27a4a6a3849e19082ecb3e40bd4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/176677
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>