intel/compiler: Request bitfield_reverse lowering on pre-Gen7 hardware

See the previous commit for the explanation of the Fixes tag.

Hurts 21 shaders in shader-db.  All of the hurt shaders are in Unreal
Engine 4 tech demos.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: 7afa26d4 ("nir: Add lowering for nir_op_bitfield_reverse.")
(cherry picked from commit b418269d)
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
[Juan A. Suarez: resolve trivial conflicts]

Conflicts:
	src/intel/compiler/brw_compiler.c