x86: fix pcmpestrm and pcmpistrm

Fix obvious typos (decrement and off-by-one error) in pcmpestrm and pcmpistrm
which resulted in infinite loop. Reported by Frank Mehnert,
spotted also by Coverity (bug 84752853).

Reported-by: Frank Mehnert <frank.mehnert@oracle.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
