cmd/compile: mark BS{F,R}L as clobbering flags

Turns out it doesn't currently matter, as these ops are always issued
together with a BTSL which does clobber flags. So I can't write a test
that currently fails. But better to be future-proof.

BS{F,R}Q generates flags, so it doesn't need to be marked as clobbering.

Change-Id: I70daea154023fd435fac696bf3a384803c647cd3
Reviewed-on: https://go-review.googlesource.com/117375


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>