The algorithms used by this pass, especially for division, are heavily
based on the work Ian Romanick did for the similar int64 lowering pass
in the GLSL compiler.

v2: Properly handle vectors

v3: Get rid of log2_denom stuff.  Since we're using bcsel, we do all the
    calculations anyway and this is just extra instructions.

v4:
 - Add back in the log2_denom stuff since it's needed for ensuring that
   the shifts don't overflow.
 - Rework the looping part of the pass to be easier to expand.

Reviewed-by: Matt Turner <mattst88@gmail.com>