spirv: Re-emit constants at their uses

spirv: Re-emit constants at their uses

Right now, spirv_to_nir places all constants at the top of the function
and has a hash table to de-duplicate them.  This change drops the hash
table and starts re-emitting constants more-or-less at their uses.  This
is more consistent with what we do in GLSL -> NIR translation.  It is,
however, a change to SPIR-V -> NIR translation which will likely affect
other optimizations in unexpected ways so it should be evaluated
separately.

This gives some good saves for spills/fills for Intel, without causing
any significant regressions on RADV, because it is using the
nir_opt_reuse_constants() pass.  In the long run that should be superseded
by some form of GCM.

```
Intel TGL results for Intel fossils:

Totals:
Instrs: 183287977 -> 183269431 (-0.01%); split: -0.07%, +0.06%
Cycles: 18224600804 -> 18223114114 (-0.01%); split: -0.16%, +0.15%
Spill count: 111031 -> 108377 (-2.39%); split: -2.45%, +0.06%
Fill count: 221781 -> 216479 (-2.39%); split: -2.39%, +0.00%
Scratch Memory Size: 4355072 -> 4180992 (-4.00%); split: -5.31%, +1.32%

Totals from 43684 (6.54% of 667704) affected shaders:
Instrs: 38289482 -> 38270936 (-0.05%); split: -0.33%, +0.28%
Cycles: 7166415272 -> 7164928582 (-0.02%); split: -0.40%, +0.38%
Spill count: 93747 -> 91093 (-2.83%); split: -2.90%, +0.07%
Fill count: 190943 -> 185641 (-2.78%); split: -2.78%, +0.00%
Scratch Memory Size: 3127296 -> 2953216 (-5.57%); split: -7.40%, +1.83%
```

```
RADV GFX1100 results for radv fossils:

Totals:
Instrs: 71623708 -> 71624667 (+0.00%); split: -0.00%, +0.01%
CodeSize: 369324312 -> 369334744 (+0.00%); split: -0.00%, +0.01%
SpillSGPRs: 13586 -> 13582 (-0.03%)
SpillVGPRs: 911 -> 910 (-0.11%); split: -0.55%, +0.44%
Latency: 632887831 -> 632880378 (-0.00%); split: -0.00%, +0.00%
InvThroughput: 81674859 -> 81676684 (+0.00%); split: -0.00%, +0.01%
VClause: 1273752 -> 1273727 (-0.00%); split: -0.00%, +0.00%
SClause: 2409593 -> 2409078 (-0.02%); split: -0.02%, +0.00%
Copies: 4063579 -> 4064425 (+0.02%); split: -0.05%, +0.07%
Branches: 1196723 -> 1196720 (-0.00%); split: -0.00%, +0.00%

Totals from 16244 (12.17% of 133461) affected shaders:
Instrs: 31116807 -> 31117766 (+0.00%); split: -0.01%, +0.01%
CodeSize: 160316656 -> 160327088 (+0.01%); split: -0.01%, +0.01%
SpillSGPRs: 12270 -> 12266 (-0.03%)
SpillVGPRs: 835 -> 834 (-0.12%); split: -0.60%, +0.48%
Latency: 344388549 -> 344381096 (-0.00%); split: -0.01%, +0.00%
InvThroughput: 43043761 -> 43045586 (+0.00%); split: -0.01%, +0.01%
VClause: 433221 -> 433196 (-0.01%); split: -0.01%, +0.00%
SClause: 900825 -> 900310 (-0.06%); split: -0.06%, +0.00%
Copies: 1989000 -> 1989846 (+0.04%); split: -0.11%, +0.15%
Branches: 676625 -> 676622 (-0.00%); split: -0.00%, +0.00%
```

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5282>