agx: Coalesce more collects

Try harder to coalesce collects, by trying to allocate collects only to regions of the register file where we actually have a full vector worth of registers free. If we already know that the vector will be blocked later, it's not a good base register to pick since we'd be force to shuffle later. So, this tweak to the collect coalescing heuristic lets us eliminate a pile of pointless copying.

shader-db results are excellent. Note that, although we use more registers, none of the shaders tested had their thread count affected, likely because the max HURT isn't too high and most of the scary % here is from using a few more registers when the register pressure is already low. In the near future, that property will become guaranteed thanks to live range splitting, too.

total instructions in shared programs: 1507337 -> 1500562 (-0.45%)
instructions in affected programs: 428137 -> 421362 (-1.58%)
helped: 2658
HURT: 167
helped stats (abs) min: 1.0 max: 34.0 x̄: 2.63 x̃: 2
helped stats (rel) min: 0.10% max: 25.00% x̄: 3.04% x̃: 2.14%
HURT stats (abs)   min: 1.0 max: 10.0 x̄: 1.24 x̃: 1
HURT stats (rel)   min: 0.20% max: 23.81% x̄: 3.90% x̃: 3.57%
95% mean confidence interval for instructions value: -2.49 -2.31
95% mean confidence interval for instructions %-change: -2.76% -2.51%
Instructions are helped.

total bytes in shared programs: 10333670 -> 10293172 (-0.39%)
bytes in affected programs: 2996682 -> 2956184 (-1.35%)
helped: 2660
HURT: 175
helped stats (abs) min: 2.0 max: 204.0 x̄: 15.70 x̃: 12
helped stats (rel) min: 0.08% max: 23.08% x̄: 2.64% x̃: 1.83%
HURT stats (abs)   min: 2.0 max: 60.0 x̄: 7.26 x̃: 6
HURT stats (rel)   min: 0.12% max: 22.39% x̄: 3.19% x̃: 2.78%
95% mean confidence interval for bytes value: -14.81 -13.76
95% mean confidence interval for bytes %-change: -2.39% -2.18%
Bytes are helped.

total halfregs in shared programs: 417284 -> 427363 (2.42%)
halfregs in affected programs: 49814 -> 59893 (20.23%)
helped: 95
HURT: 3018
helped stats (abs) min: 1.0 max: 8.0 x̄: 2.29 x̃: 2
helped stats (rel) min: 2.44% max: 28.57% x̄: 9.20% x̃: 6.06%
HURT stats (abs)   min: 1.0 max: 14.0 x̄: 3.41 x̃: 4
HURT stats (rel)   min: 2.08% max: 150.00% x̄: 36.54% x̃: 27.27%
95% mean confidence interval for halfregs value: 3.17 3.31
95% mean confidence interval for halfregs %-change: 34.05% 36.23%
Halfregs are HURT.

total threads in shared programs: 16465280 -> 16465280 (0.00%)
threads in affected programs: 0 -> 0
helped: 0
HURT: 0

Signed-off-by: Alyssa Rosenzweig alyssa@rosenzweig.io