ac/nir: Reuse uniforms from top part of culling shaders.

ac/nir: Reuse uniforms from top part of culling shaders.

Uniforms have the same value in all invocations, therefore they
can safely be reused by invocations even after repacking.
This saves several instructions from culling shaders, mainly
UBO loads and such.

We exclude uniform floats, because those would harm the VGPR
usage of the shaders too much.

Fossil DB results on Sienna Cichlid (with NGG culling on):

Totals from 55379 (43.05% of 128647) affected shaders:
VGPRs: 1926472 -> 1925360 (-0.06%); split: -0.07%, +0.01%
SpillSGPRs: 139 -> 330 (+137.41%)
CodeSize: 159472988 -> 157462856 (-1.26%); split: -1.27%, +0.00%
MaxWaves: 1571492 -> 1571412 (-0.01%)
Instrs: 30665685 -> 30302076 (-1.19%); split: -1.21%, +0.02%
Latency: 127385148 -> 126723891 (-0.52%); split: -0.55%, +0.03%
InvThroughput: 21096298 -> 20773069 (-1.53%); split: -1.53%, +0.00%
VClause: 514792 -> 511231 (-0.69%); split: -0.83%, +0.13%
SClause: 713959 -> 679556 (-4.82%); split: -4.84%, +0.02%
Copies: 2975106 -> 2828185 (-4.94%); split: -5.39%, +0.45%
Branches: 1201921 -> 1152766 (-4.09%)
PreSGPRs: 1753786 -> 1892848 (+7.93%); split: -0.00%, +7.93%
PreVGPRs: 1590522 -> 1583574 (-0.44%); split: -0.44%, +0.00%

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10525>