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>

src/compiler/nir/nir_opt_algebraic.py

@@ -312,6 +312,19 @@ optimizations = [
      'options->lower_unpack_snorm_4x8'),
 ]
 
+# Unreal Engine 4 demo applications open-codes bitfieldReverse()
+def bitfield_reverse(u):
+    step1 = ('ior', ('ishl', u, 16), ('ushr', u, 16))
+    step2 = ('ior', ('ishl', ('iand', step1, 0x00ff00ff), 8), ('ushr', ('iand', step1, 0xff00ff00), 8))
+    step3 = ('ior', ('ishl', ('iand', step2, 0x0f0f0f0f), 4), ('ushr', ('iand', step2, 0xf0f0f0f0), 4))
+    step4 = ('ior', ('ishl', ('iand', step3, 0x33333333), 2), ('ushr', ('iand', step3, 0xcccccccc), 2))
+    step5 = ('ior', ('ishl', ('iand', step4, 0x55555555), 1), ('ushr', ('iand', step4, 0xaaaaaaaa), 1))
+
+    return step5
+
+optimizations += [(bitfield_reverse('x'), ('bitfield_reverse', 'x'))]
+
+
 # Add optimizations to handle the case where the result of a ternary is
 # compared to a constant.  This way we can take things like
 #