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.

Cc: Stephen Checkoway <s@pahtak.org>
Cc: Tim Northover <t.p.northover@gmail.com>
Cc: Jim Grosbach <grosbach@apple.com>
Cc: Chris Lattner <sabre@nondot.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>