i965/fs: Add infrastructure for generating CSEL instructions.

i965/fs: Add infrastructure for generating CSEL instructions.

v2 (idr): Don't allow CSEL with a non-float src2.

v3 (idr): Add CSEL to fs_inst::flags_written.  Suggested by Matt.

v4 (idr): Only set BRW_ALIGN_16 on Gen < 10 (suggested by Matt).  Don't
reset the access mode afterwards (suggested by Samuel and Matt).  Add
support for CSEL not modifying the flags to more places (requested by
Matt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com> [v3]
Reviewed-by: Matt Turner <mattst88@gmail.com>