aco: use MUBUF in some situations instead of splitting vertex fetches

aco: use MUBUF in some situations instead of splitting vertex fetches

Fixes most of the regressions from splitting vertex fetches in an earlier
commit.

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 0 -> 0 (0.00 %)
VGPRS: 0 -> 0 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 0 -> 0 (0.00 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 0 -> 0 (0.00 %)

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 562696 -> 558344 (-0.77 %)
VGPRS: 395596 -> 393752 (-0.47 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 11600912 -> 11311804 (-2.49 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 101839 -> 102372 (0.52 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3086>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3086>