aco: Reassign dead definitions of p_split_vector to associated register

aco: Reassign dead definitions of p_split_vector to associated register

Any unused split_vector definition can always use the same register
as the operand. This avoids creating unnecessary copies.

Fossil DB stats on Rembrandt (RDNA2):
Totals from 3904 (2.89% of 134906) affected shaders:
CodeSize: 18326692 -> 18271688 (-0.30%)
Instrs: 3386632 -> 3372888 (-0.41%)
Latency: 42337481 -> 42330085 (-0.02%); split: -0.02%, +0.00%
InvThroughput: 6566731 -> 6566424 (-0.00%); split: -0.01%, +0.00%
Copies: 224301 -> 210559 (-6.13%)

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16161>