tu: Initial link-time optimizations

This is mostly taken from radv, and cleaned up a bit: don't explicitly list every stage at the beginning, and name the shaders "producer" and "consumer" to reduce confusion. I also stripped out a lot of other stuff to get to the bare minimum of calling nir_link_opt_varyings, nir_remove_unused_varyings, and nir_compact_varyings and then cleaning up the fallout. In the future we may want to temporarily scalarize I/O like radv does, and add back a few things like the psize optimization. In the meantime this already provides a lot of benefit.

Results from the radv fossil_db with some apps not compilable by turnip removed:

Totals:
MaxWaves: 1637288 -> 1668200 (+1.89%); split: +1.89%, -0.00%
Instrs: 54620287 -> 54114442 (-0.93%); split: -0.98%, +0.05%
CodeSize: 92235646 -> 91277584 (-1.04%); split: -1.07%, +0.03%
NOPs: 11176775 -> 11185206 (+0.08%); split: -0.63%, +0.71%
Full: 1689271 -> 1657175 (-1.90%); split: -1.92%, +0.02%
(ss): 1318763 -> 1317757 (-0.08%); split: -1.40%, +1.32%
(sy): 618795 -> 617724 (-0.17%); split: -0.70%, +0.53%
(ss)-stall: 3496370 -> 3470116 (-0.75%); split: -1.37%, +0.62%
(sy)-stall: 23512954 -> 23511164 (-0.01%); split: -1.04%, +1.03%
STPs: 27557 -> 27461 (-0.35%)
LDPs: 22948 -> 22804 (-0.63%)
Cat0: 11823765 -> 11829681 (+0.05%); split: -0.62%, +0.67%
Cat1: 3120042 -> 2991831 (-4.11%); split: -4.43%, +0.32%
Cat2: 28605309 -> 28324829 (-0.98%); split: -0.98%, +0.00%
Cat3: 7334628 -> 7252342 (-1.12%); split: -1.12%, +0.00%
Cat4: 1216514 -> 1204894 (-0.96%)
Cat5: 863976 -> 861926 (-0.24%)
Cat6: 1648571 -> 1641457 (-0.43%)

Totals from 23575 (16.16% of 145856) affected shaders:
MaxWaves: 258806 -> 289718 (+11.94%); split: +11.94%, -0.00%
Instrs: 7571190 -> 7065345 (-6.68%); split: -7.04%, +0.36%
CodeSize: 13864308 -> 12906246 (-6.91%); split: -7.09%, +0.18%
NOPs: 959185 -> 967616 (+0.88%); split: -7.35%, +8.23%
Full: 313335 -> 281239 (-10.24%); split: -10.36%, +0.11%
(ss): 154628 -> 153622 (-0.65%); split: -11.90%, +11.25%
(sy): 69758 -> 68687 (-1.54%); split: -6.21%, +4.67%
(ss)-stall: 322002 -> 295748 (-8.15%); split: -14.92%, +6.76%
(sy)-stall: 3270366 -> 3268576 (-0.05%); split: -7.45%, +7.40%
STPs: 3624 -> 3528 (-2.65%)
LDPs: 1074 -> 930 (-13.41%)
Cat0: 1022684 -> 1028600 (+0.58%); split: -7.13%, +7.71%
Cat1: 531102 -> 402891 (-24.14%); split: -26.04%, +1.90%
Cat2: 4090309 -> 3809829 (-6.86%); split: -6.86%, +0.00%
Cat3: 1449686 -> 1367400 (-5.68%); split: -5.69%, +0.01%
Cat4: 103543 -> 91923 (-11.22%)
Cat5: 57441 -> 55391 (-3.57%)
Cat6: 316096 -> 308982 (-2.25%)