ac/nir: Set speculatable for buffer loads where allowed

ac/nir: Set speculatable for buffer loads where allowed

This brings the nir path in line with the TGSI path.

Totals from affected shaders:
SGPRS: 2984 -> 2984 (0.00 %)
VGPRS: 2792 -> 2652 (-5.01 %)
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: 247380 -> 248072 (0.28 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 121 -> 132 (9.09 %)
Wait states: 0 -> 0 (0.00 %)

Most of the change came from DiRT: Showdown, and came from sinking SSBO
loads.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>