x86: Use REP BSF unconditionally

Make "REP BSF" unconditional, as per the suggestion of hpa
and Linus, this removes the insane BSF_PREFIX conditional
and simplifies the logic.

Suggested-by: "H. Peter Anvin" <hpa@zytor.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>