nir/opt_sink: sink agx backfacing

helps an elden ring shader:

Totals from 1 (0.03% of 3206) affected shaders:
Instrs: 4146 -> 4141 (-0.12%)
CodeSize: 27374 -> 27334 (-0.15%)
ALU: 2554 -> 2549 (-0.20%)
FSCIB: 2554 -> 2549 (-0.20%)

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35559>
1 file changed