aco: fix consecutively written vgprs from vmem instructions

aco: fix consecutively written vgprs from vmem instructions

If one VMEM instruction uses a sampler and the other doesn't, we can't do
this optimization.

Totals from 47 (0.04% of 127638) affected shaders:
CodeSize: 271744 -> 271656 (-0.03%); split: -0.04%, +0.01%
Instrs: 52783 -> 52761 (-0.04%); split: -0.05%, +0.01%
Cycles: 5547040 -> 5546952 (-0.00%); split: -0.00%, +0.00%
VMEM: 10022 -> 9887 (-1.35%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4949>