radv/llvm: enable lower_unpack_half_2x16

To match ACO.

fossils-db (Navi10):
Totals from 294 (0.22% of 136420) affected shaders:
SGPRs: 16504 -> 16496 (-0.05%)
VGPRs: 19008 -> 19124 (+0.61%); split: -0.06%, +0.67%
SpillVGPRs: 511 -> 476 (-6.85%); split: -7.63%, +0.78%
CodeSize: 1688852 -> 1687932 (-0.05%); split: -0.10%, +0.05%
Scratch: 305152 -> 307200 (+0.67%)
MaxWaves: 2877 -> 2878 (+0.03%)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6937>
1 file changed