Revert "implement PR9264: disambiguating 'bt mem, imm' as a btl."

This reverts commit 824a9076eaf8d109bc79f53e51b7d7a045f42552.  It takes
the wrong approach to the problem by always disambiguating the $imm form
of bt as btl, like GNU as.  The problem will be solved correctly in the
next commit.

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>
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>