Fix clang's internal assembler adding unwanted prefix to VIA xstore
Merge commit 2d8c18fbbdd1 from llvm git (by Jessica Clarke):
[X86] Don't add implicit REP prefix to VIA PadLock xstore Commit 8fa3e8fa1492 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
(cherry picked from commit 4b9725184e315596aa401cecb230033c6d6d5f29)