aco/insert_exec_mask: Simplify WQM handling (1/2)

aco/insert_exec_mask: Simplify WQM handling (1/2)

by using p_end_wqm as indicator for when to end WQM mode.

Totals from 10049 (13.12% of 76572) affected shaders: (GFX11)

MaxWaves: 301126 -> 301136 (+0.00%)
Instrs: 7061909 -> 7049272 (-0.18%); split: -0.21%, +0.03%
CodeSize: 37720684 -> 37664244 (-0.15%); split: -0.18%, +0.03%
VGPRs: 357204 -> 357180 (-0.01%); split: -0.13%, +0.12%
Latency: 62757830 -> 62827080 (+0.11%); split: -0.06%, +0.17%
InvThroughput: 8589248 -> 8589963 (+0.01%); split: -0.02%, +0.02%
VClause: 132541 -> 132547 (+0.00%); split: -0.03%, +0.03%
SClause: 322916 -> 322964 (+0.01%); split: -0.04%, +0.05%
Copies: 546446 -> 547657 (+0.22%); split: -0.13%, +0.35%
Branches: 189527 -> 188293 (-0.65%)
PreSGPRs: 332792 -> 332529 (-0.08%); split: -0.08%, +0.00%
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25038>