More tweaks to X86 instructions to allow the 'w' suffix in places it makes

sense, when the instruction takes the 16-bit ax register or m16 memory
location.  These changes to llvm-mc matches what the darwin assembler allows
for these instructions.  Also added the missing flex (without the wait prefix)
and ud2a as an alias to ud2 (still to add ud2b).

llvm-svn: 117031