ac/nir: Use es_accepted variable after culling.

ac/nir: Use es_accepted variable after culling.

This avoids re-calculating the exec mask for ES vertices,
and makes it unnecessary to count the number of vertices left.

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

Totals from 58239 (45.27% of 128647) affected shaders:
CodeSize: 166521108 -> 166356072 (-0.10%); split: -0.10%, +0.00%
Instrs: 31961308 -> 31920041 (-0.13%); split: -0.13%, +0.00%
Latency: 138820463 -> 138815742 (-0.00%); split: -0.04%, +0.04%
InvThroughput: 22460177 -> 22459553 (-0.00%); split: -0.00%, +0.00%
SClause: 753744 -> 753746 (+0.00%)
Copies: 3093140 -> 3226647 (+4.32%); split: -0.03%, +4.34%

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>