agx: Coalesce collects when possible

agx: Coalesce collects when possible

Track collects and use them as affinities when choosing registers. On
glmark2:

total instructions in shared programs: 5498 -> 5388 (-2.00%)
instructions in affected programs: 2748 -> 2638 (-4.00%)
helped: 31
HURT: 0
helped stats (abs) min: 1.0 max: 12.0 x̄: 3.55 x̃: 3
helped stats (rel) min: 0.09% max: 57.14% x̄: 10.58% x̃: 5.97%
95% mean confidence interval for instructions value: -4.61 -2.49
95% mean confidence interval for instructions %-change: -15.16% -6.00%
Instructions are helped.

total bytes in shared programs: 37280 -> 36620 (-1.77%)
bytes in affected programs: 18880 -> 18220 (-3.50%)
helped: 31
HURT: 0
helped stats (abs) min: 6.0 max: 72.0 x̄: 21.29 x̃: 18
helped stats (rel) min: 0.07% max: 48.98% x̄: 9.16% x̃: 5.17%
95% mean confidence interval for bytes value: -27.64 -14.94
95% mean confidence interval for bytes %-change: -13.03% -5.29%
Bytes are helped.

total halfregs in shared programs: 1267 -> 1279 (0.95%)
halfregs in affected programs: 37 -> 49 (32.43%)
helped: 0
HURT: 9
HURT stats (abs)   min: 1.0 max: 2.0 x̄: 1.33 x̃: 1
HURT stats (rel)   min: 16.67% max: 66.67% x̄: 35.58% x̃: 28.57%
95% mean confidence interval for halfregs value: 0.95 1.72
95% mean confidence interval for halfregs %-change: 21.50% 49.67%
Halfregs are HURT.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19590>