intel/fs: Pick the lowest register pressure schedule when spilling

intel/fs: Pick the lowest register pressure schedule when spilling

We try various pre-RA scheduler modes and see if any of them allow
us to register allocate without spilling.  If all of them spill,
however, we left it on the last mode: LIFO.  This is unfortunately
sometimes significantly worse than other modes (such as "none").

This patch makes us instead select the pre-RA scheduling mode that
gives the lowest register pressure estimate, if none of them manage
to avoid spilling.  The hope is that this scheduling will spill the
least out of all of them.

fossil-db stats (on Alchemist) speak for themselves:

    Totals:
    Instrs: 197297092 -> 195326552 (-1.00%); split: -1.02%, +0.03%
    Cycles: 14291286956 -> 14303502596 (+0.09%); split: -0.55%, +0.64%
    Spill count: 190886 -> 129204 (-32.31%); split: -33.01%, +0.70%
    Fill count: 361408 -> 225038 (-37.73%); split: -39.17%, +1.43%
    Scratch Memory Size: 12935168 -> 10868736 (-15.98%); split: -16.08%, +0.10%

    Totals from 1791 (0.27% of 668386) affected shaders:
    Instrs: 7628929 -> 5658389 (-25.83%); split: -26.50%, +0.67%
    Cycles: 719326691 -> 731542331 (+1.70%); split: -10.95%, +12.65%
    Spill count: 110627 -> 48945 (-55.76%); split: -56.96%, +1.20%
    Fill count: 221560 -> 85190 (-61.55%); split: -63.89%, +2.34%
    Scratch Memory Size: 4471808 -> 2405376 (-46.21%); split: -46.51%, +0.30%

Improves performance when using XeSS in Cyberpunk 2077 by 90% on A770.
Improves performance of Borderlands 3 by 1.54% on A770.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24707>