The current version of LLVM X86 disassembler incorrectly interprets some...

The current version of LLVM X86 disassembler incorrectly interprets some possible sets of x86 prefixes. This patch is the first step to close PR7709 and PR17697. There will be next patch(es) to close relative PRs.
Differential Revision: https://reviews.llvm.org/D36788

M    lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
M    lib/Target/X86/Disassembler/X86DisassemblerDecoder.h
A    test/MC/Disassembler/X86/prefixes-i386.s
A    test/MC/Disassembler/X86/prefixes-x86_64.s
M    test/MC/Disassembler/X86/prefixes.txt

llvm-svn: 311882