Pull in r322623 from upstream llvm trunk (by Andrew V. Tischenko):

Allow usage of X86-prefixes as separate instrs.
Differential Revision: https://reviews.llvm.org/D42102

This should fix parse errors when x86 prefixes (such as 'lock' and
'rep') are followed by various non-mnemonic tokens, e.g. comments, .byte
directives and labels.

PR: 224669,225054