Revert the patches adding a popcount loop idiom recognition pass.

There are still bugs in this pass, as well as other issues that are
being worked on, but the bugs are crashers that occur pretty easily in
the wild. Test cases have been sent to the original commit's review
thread.

This reverts the commits:
  r169671: Fix a logic error.
  r169604: Move the popcnt tests to an X86 subdirectory.
  r168931: Initial commit adding the pass.

llvm-svn: 169683