aco: create better code for boolean phis with constant operands

aco: create better code for boolean phis with constant operands

fossil-db (Navi):
Totals from 6394 (4.70% of 135946) affected shaders:
SGPRs: 651408 -> 651344 (-0.01%)
SpillSGPRs: 52102 -> 52019 (-0.16%)
CodeSize: 68369664 -> 68229180 (-0.21%); split: -0.21%, +0.00%
Instrs: 13236611 -> 13202126 (-0.26%); split: -0.26%, +0.00%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3388>