st/nir: Don't lower indirects when linking

st/nir: Don't lower indirects when linking

I believe this was stuck here early because otherwise
nir_opt_copy_prop_vars could undo what lower_io_to_temporaries does.
However that has since been fixed. Also, we now use scratch for large
variables so the comment is stale.

On radeonsi these are the shader-db results:

Totals:
SGPRS: 3955968 -> 3955968 (0.00 %)
VGPRS: 2220208 -> 2220220 (0.00 %)
Spilled SGPRs: 11387 -> 11387 (0.00 %)
Spilled VGPRs: 97 -> 97 (0.00 %)
Private memory VGPRs: 2528 -> 2528 (0.00 %)
Scratch size: 2656 -> 2656 (0.00 %) dwords per thread
Code Size: 76002108 -> 76002204 (0.00 %) bytes
LDS: 740 -> 740 (0.00 %) blocks
Max Waves: 772779 -> 772776 (-0.00 %)
Wait states: 0 -> 0 (0.00 %)

Totals from affected shaders:
SGPRS: 176 -> 176 (0.00 %)
VGPRS: 144 -> 156 (8.33 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 12104 -> 12200 (0.79 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 28 -> 25 (-10.71 %)
Wait states: 0 -> 0 (0.00 %)

The few small regressions are due to nir_opt_large_constants kicking in
when indirect lowering happens to result in smaller code after
optimization since the array is very simple.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>