nir/algrbraic: Don't optimize open-coded bitfield reverse when lowering is enabled

This caused a problem on Sandybridge where an open-coded
bitfieldReverse() function could be optimized to a
nir_op_bitfield_reverse that would generate an unsupported BFREV
instruction in the backend.  This was encountered in some Unreal4 tech
demos in shader-db.  The bug was not previously noticed because we don't
actually try to run those demos on Sandybridge.

The fixes tag is a bit a lie.  The actual bug was introduced about
26,000 commits earlier in 371c4b3c48 ("nir: Recognize open-coded
bitfield_reverse.").  Without the NIR lowering pass, the flag needed to
avoid the optimization does not exist.  Hopefully nobody will care to
fix this on an earlier Mesa release.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 7afa26d4e3 ("nir: Add lowering for nir_op_bitfield_reverse.")

src/compiler/nir/nir_opt_algebraic.py

@@ -1319,7 +1319,7 @@ def bitfield_reverse(u):
 
     return step5
 
-optimizations += [(bitfield_reverse('x@32'), ('bitfield_reverse', 'x'))]
+optimizations += [(bitfield_reverse('x@32'), ('bitfield_reverse', 'x'), '!options->lower_bitfield_reverse')]
 
 # For any float comparison operation, "cmp", if you have "a == a && a cmp b"
 # then the "a == a" is redundant because it's equivalent to "a is not NaN"