aco: don't insert a copy when emitting p_wqm

aco: don't insert a copy when emitting p_wqm

Totals from 351 (0.46% of 76572) affected shaders: (GFX11)

Instrs: 709202 -> 709600 (+0.06%); split: -0.02%, +0.08%
CodeSize: 3606364 -> 3608040 (+0.05%); split: -0.01%, +0.06%
Latency: 3589841 -> 3590756 (+0.03%); split: -0.01%, +0.03%
InvThroughput: 463303 -> 463324 (+0.00%)
SClause: 28147 -> 28201 (+0.19%); split: -0.02%, +0.22%
Copies: 43243 -> 43204 (-0.09%); split: -0.24%, +0.15%
PreSGPRs: 21028 -> 21042 (+0.07%)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25038>