aco: don't create phis with undef operands in the boolean phi pass

aco: don't create phis with undef operands in the boolean phi pass

We can create better merge code is we pass on undef.

fossil-db (Navi):
Totals from 1208 (0.89% of 135946) affected shaders:
SGPRs: 66864 -> 66200 (-0.99%); split: -1.04%, +0.05%
SpillSGPRs: 1179 -> 1156 (-1.95%)
CodeSize: 6516672 -> 6469564 (-0.72%); split: -0.76%, +0.04%
Instrs: 1232680 -> 1220859 (-0.96%); split: -0.97%, +0.01%

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>