[X86][Disassembler] Fix LOCK prefix disassembler support

Summary:
If LOCK prefix is not the first prefix in an instruction, LLVM
disassembler silently drops the prefix.

The fix is to select a proper instruction with a builtin LOCK prefix if
one exists.

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49001

llvm-svn: 336400