agx: Allow uniform sources on phis

agx: Allow uniform sources on phis

The parallel copy lowering has been able to handle uniform sources since 98f0ebf2647 ("agx:
Pass agx_index to agx_copy"), and uniform sources work fine with phis. It's not
super common but there's no need to restrict them. This is a small instruction
count win and will greatly simplify the lowering later in this series.

total instructions in shared programs: 1523806 -> 1523285 (-0.03%)
instructions in affected programs: 17088 -> 16567 (-3.05%)
helped: 38
HURT: 1
helped stats (abs) min: 1.0 max: 44.0 x̄: 13.95 x̃: 7
helped stats (rel) min: 0.42% max: 18.64% x̄: 4.73% x̃: 1.26%
HURT stats (abs)   min: 9.0 max: 9.0 x̄: 9.00 x̃: 9
HURT stats (rel)   min: 8.57% max: 8.57% x̄: 8.57% x̃: 8.57%
95% mean confidence interval for instructions value: -17.95 -8.77
95% mean confidence interval for instructions %-change: -6.35% -2.43%
Instructions are helped.

total bytes in shared programs: 10447658 -> 10444532 (-0.03%)
bytes in affected programs: 118850 -> 115724 (-2.63%)
helped: 38
HURT: 1
helped stats (abs) min: 6.0 max: 264.0 x̄: 83.68 x̃: 45
helped stats (rel) min: 0.36% max: 16.51% x̄: 4.14% x̃: 1.09%
HURT stats (abs)   min: 54.0 max: 54.0 x̄: 54.00 x̃: 54
HURT stats (rel)   min: 7.30% max: 7.30% x̄: 7.30% x̃: 7.30%
95% mean confidence interval for bytes value: -107.68 -52.62
95% mean confidence interval for bytes %-change: -5.55% -2.13%
Bytes are helped.

total halfregs in shared programs: 419446 -> 419444 (<.01%)
halfregs in affected programs: 29 -> 27 (-6.90%)
helped: 1
HURT: 0

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21122>