nir/gather_info: fix per-vertex handling in try_mask_partial_io

nir/gather_info: fix per-vertex handling in try_mask_partial_io

pipeline-db (Navi, ACO):
Totals from affected shaders:
SGPRS: 6432 -> 6432 (0.00 %)
VGPRS: 11924 -> 11924 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Scratch size: 1596 -> 1596 (0.00 %) dwords per thread
Code Size: 575524 -> 518620 (-9.89 %) bytes
LDS: 12187 -> 12187 (0.00 %) blocks
Max Waves: 2695 -> 2695 (0.00 %)

Helps a few hundred Dark Souls 3 shaders.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4190>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4190>