aco: make p_wqm a marker instruction without Operands/Definitions

aco: make p_wqm a marker instruction without Operands/Definitions

Totals from 28277 (36.93% of 76572) affected shaders: (GFX11)

MaxWaves: 833930 -> 833898 (-0.00%); split: +0.01%, -0.01%
Instrs: 21366950 -> 21353346 (-0.06%); split: -0.11%, +0.05%
CodeSize: 112855368 -> 112610508 (-0.22%); split: -0.24%, +0.03%
VGPRs: 1157748 -> 1158540 (+0.07%); split: -0.10%, +0.17%
SpillSGPRs: 2465 -> 2463 (-0.08%); split: -0.16%, +0.08%
Latency: 168339886 -> 168383646 (+0.03%); split: -0.10%, +0.12%
InvThroughput: 25164895 -> 25158376 (-0.03%); split: -0.08%, +0.06%
VClause: 347660 -> 346256 (-0.40%); split: -0.55%, +0.15%
SClause: 794460 -> 799521 (+0.64%); split: -0.33%, +0.97%
Copies: 1151908 -> 1148370 (-0.31%); split: -0.54%, +0.23%
Branches: 359447 -> 359437 (-0.00%); split: -0.01%, +0.00%
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25038>