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anv: Set shader_spilling_rate=15 by default

Matt Turner requested to merge mattst88/mesa:default_shader_spilling_rate into main

This avoids massively long shader compile times when there is lots of spilling, at a minor cost of a few more spills/fills. Choose 15 as it is already the default used by the Cyberpunk 2077 driconf workaround.

Surprisingly the number of additional spills/fills are miniscule in fossil-db:

  Instructions in all programs: 152680595 -> 152681525 (+0.0%)
  SENDs in all programs: 7672789 -> 7672789 (+0.0%)
  Loops in all programs: 48469 -> 48469 (+0.0%)
  Cycles in all programs: 11981743456 -> 11984228708 (+0.0%)
  Spills in all programs: 42989 -> 42779 (-0.5%)
  Fills in all programs: 76380 -> 76776 (+0.5%)

partly because of the chaotic unpredictability that the choice of register to spill has on a shader. For example, this patch massively helps some shaders in terms of spills/fills:

  Spills helped fossils/fossil-db/steam-native/red_dead_redemption2.vk-g6.foz/4101ff9c9b83bf22/SIMD8 fragment: 3208 -> 2894 (-9.8%)
  Fills helped fossils/fossil-db/steam-native/red_dead_redemption2.vk-g6.foz/4101ff9c9b83bf22/SIMD8 fragment: 7258 -> 6795 (-6.4%)
  Spills helped fossils/q2rtx/q2rtx-rt-pipeline.976f4ab1c0fee975.1.foz/c496e8a549f6b4bf/compute: 109 -> 92 (-15.6%)
Edited by Matt Turner

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