aco: coalesce parallelcopies during register allocation

aco: coalesce parallelcopies during register allocation

These are the result of lowering to CSSA, and should be removed if possible

Totals from affected shaders: (VEGA)
SGPRS: 544544 -> 544544 (0.00 %)
VGPRS: 418224 -> 418224 (0.00 %)
Spilled SGPRs: 141826 -> 141826 (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: 65853740 -> 64703380 (-1.75 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 13669 -> 13669 (0.00 %)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4952>