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

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117031 91177308-0d34-0410-b5e6-96231b3b80d8