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

Commi

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

MFC after: 3 days

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