nir_to_tgsi: Add support for TXP.
This doesn't perfectly emit all the TXPs we might be able to (if you get any non-TXP-able projectors, that whole sampler dim gets lowered), but it should be a big boost to i915g. softpipe shader-db: total instructions in shared programs: 2953625 -> 2951201 (-0.08%) instructions in affected programs: 108901 -> 106477 (-2.23%) total temps in shared programs: 565869 -> 566632 (0.13%) temps in affected programs: 8922 -> 9685 (8.55%) i915g shader-db: total instructions in shared programs: 372570 -> 370254 (-0.62%) instructions in affected programs: 39825 -> 37509 (-5.82%) total tex_indirect in shared programs: 11420 -> 11074 (-3.03%) tex_indirect in affected programs: 913 -> 567 (-37.90%) LOST: 0 GAINED: 12 Closes: #4984 Reviewed-by:Adam Jackson <ajax@redhat.com> Part-of: <mesa/mesa!11912>