[PATCH 2/2] X86: infer immediate forms of bit-test instructions

The instruction mnemonics for the immediate forms of bit-test
instructions including bt, btr and bts, btc do not work. llvm-mc barfs
with:

  error: ambiguous instructions require an explicit suffix

This is highly user-unfriendly, since we can easily infer what the user
meant by inspecting $imm and translating the instruction appropriately.
Do it exactly as the Intel manual describes. Note that we are the first
major assembler to do this properly: neither GNU as nor NASM does the
right thing currently.