aco/lower_phis: optimize loop exit phis

aco/lower_phis: optimize loop exit phis

This optimization works by ensuring that disabled lanes
are zero'd before any merge sequence.

Totals from 6075 (4.05% of 150170) affected shaders: (GFX10.3)
CodeSize: 57913908 -> 57913212 (-0.00%)
Instrs: 11055852 -> 11055678 (-0.00%)
Latency: 438705219 -> 438534652 (-0.04%); split: -0.04%, +0.00%
InvThroughput: 125284101 -> 125251397 (-0.03%); split: -0.03%, +0.00%
Copies: 807388 -> 821035 (+1.69%); split: -0.00%, +1.69%
Branches: 391827 -> 391782 (-0.01%)
PreSGPRs: 574841 -> 574838 (-0.00%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11659>