ac/nir: remove num_es_threads_var

ac/nir: remove num_es_threads_var

A bit count of es_accepted works for both when ngg is and isn't
dynamically enabled. Unlike the other sequence, this should only be a
single SALU instruction.

fossil-db (gfx1100, nggc):
Totals from 41388 (30.75% of 134574) affected shaders:
Instrs: 25783544 -> 25432959 (-1.36%); split: -1.36%, +0.00%
CodeSize: 127281160 -> 125878820 (-1.10%); split: -1.10%, +0.00%
Latency: 92849566 -> 92723047 (-0.14%); split: -0.14%, +0.00%
InvThroughput: 9542194 -> 9485012 (-0.60%); split: -0.60%, +0.00%
Copies: 2031074 -> 1928796 (-5.04%); split: -5.04%, +0.00%
Branches: 642407 -> 642409 (+0.00%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20321>