nir: Recognize open-coded bitfield_reverse.

nir: Recognize open-coded bitfield_reverse.

Helps 11 shaders in UnrealEngine4 demos.

I seriously hope they would have given us bitfieldReverse() if we
exposed GL 4.0 (but we do expose ARB_gpu_shader5, so why not use that
anyway?).

instructions in affected programs: 4875 -> 4633 (-4.96%)
cycles in affected programs: 270516 -> 244516 (-9.61%)

I suspect there's a *lot* of room to improve nir_search/opt_algebraic's
handling of this. We'd actually like to match, e.g., step2 by matching
step1 once and then doing a pointer comparison for the second instance
of step1, but unfortunately we generate an enormous tuple for instead.

The .text size increases by 6.5% and the .data by 17.5%.

   text     data  bss    dec    hex  filename
  22957    45224    0  68181  10a55  nir_libnir_la-nir_opt_algebraic.o
  24461    53160    0  77621  12f35  nir_libnir_la-nir_opt_algebraic.o

I'd be happy to remove this if Unreal4 uses bitfieldReverse() if it is
in a GL 4.0 context once we expose GL 4.0.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>