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 371c4b3c ("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: 7afa26d4 ("nir: Add lowering for nir_op_bitfield_reverse.")
(cherry picked from commit d3fd1c76)