aco: optimize boolean phis with uniform selections

aco: optimize boolean phis with uniform selections

Even though the boolean can be divergent, the control flow can be (at
least partially) uniform. For example, we don't have to create any
s_andn2_b64/s_and_b64/s_or_b64 instructions with this code:
a = ...
loop {
    b = bool_phi a, c
    if (uniform)
        break
    c = ...
}
d = phi c

fossil-db (Navi):
Totals from 5506 (4.05% of 135946) affected shaders:
SGPRs: 605720 -> 604024 (-0.28%)
SpillSGPRs: 52025 -> 51733 (-0.56%)
CodeSize: 65221188 -> 64957808 (-0.40%); split: -0.41%, +0.00%
Instrs: 12637881 -> 12584610 (-0.42%); split: -0.42%, +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>