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: Chris Lattner <sabre@nondot.org>
Cc: Stephen Checkoway <s@pahtak.org>
Cc: Tim Northover <t.p.northover@gmail.com>
Cc: Jim Grosbach <grosbach@apple.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
X86: infer immediate forms of bit-test instructions

WIP!

The instruction mnemonics btr and bts are perfectly valid, and have
existed since Intel 386.  GNU as supports them fine, and there are
instances of its usage in linux.git.  Unfortunately, LLVM does not
support them, and barfs with:

  error: ambiguous instructions require an explicit suffix

Fix this problem by disambiguating it just like GNU as does, following
he example set by 824a907.

To verify that this patch adheres to the behavior of GNU as, feed it
this input:

  btrl	$1, 0
  btr	$1, 0
  btsl	$1, 0
  bts	$1, 0

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>