ac/nir: Use gs_accepted variable after culling.

ac/nir: Use gs_accepted variable after culling.

This prevents us from recalculating the EXEC mask later
in the shader, and removes the requirement for
counting the number of primitives.

The stats are better than expected because they also
show that some code that is still there is now DCE'd by ACO.

Fossil DB results on Sienna Cichlid (with NGGC on):

Totals from 58239 (45.27% of 128647) affected shaders:
SpillSGPRs: 330 -> 340 (+3.03%)
CodeSize: 166356072 -> 162805724 (-2.13%)
Instrs: 31920041 -> 31089256 (-2.60%)
Latency: 138815742 -> 138113669 (-0.51%); split: -0.54%, +0.03%
InvThroughput: 22459553 -> 22404840 (-0.24%); split: -0.26%, +0.02%
SClause: 753746 -> 753765 (+0.00%); split: -0.00%, +0.01%
Copies: 3226647 -> 3268973 (+1.31%); split: -0.45%, +1.76%
Branches: 1223441 -> 1223440 (-0.00%); split: -0.00%, +0.00%
PreSGPRs: 2025339 -> 2091013 (+3.24%)

No Fossil DB changes with NGGC off.

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/11908>