[X86] Don't add implicit REP prefix to VIA PadLock xstore

Commit 8fa3e8fa added an implicit REP prefix to all VIA PadLock
instructions, but GNU as doesn't add one to xstore, only all the others.
This resulted in a kernel panic regression in FreeBSD upon updating to
LLVM 11 (https://bugs.freebsd.org/259218) which includes the commit in
question. This partially reverts that commit.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D112355