aco: improve readfirstlane after uniform ssbo loads on GFX7

aco: improve readfirstlane after uniform ssbo loads on GFX7

pipeline-db changes for GFX7:

80310 shaders in 40472 tests
Totals:
SGPRS: 3655900 -> 3643916 (-0.33 %)
VGPRS: 2678324 -> 2686324 (0.30 %)
Spilled SGPRs: 1730 -> 1634 (-5.55 %)
Spilled VGPRs: 14 -> 21 (50.00 %)
Scratch size: 15540 -> 15536 (-0.03 %) dwords per thread
Code Size: 136106120 -> 135457616 (-0.48 %) bytes
LDS: 1259 -> 1259 (0.00 %) blocks
Max Waves: 601014 -> 600206 (-0.13 %)

Totals from affected shaders:
SGPRS: 307832 -> 295848 (-3.89 %)
VGPRS: 267864 -> 275864 (2.99 %)
Spilled SGPRs: 770 -> 674 (-12.47 %)
Spilled VGPRs: 14 -> 21 (50.00 %)
Scratch size: 16 -> 12 (-25.00 %) dwords per thread
Code Size: 22007488 -> 21358984 (-2.95 %) bytes
LDS: 65 -> 65 (0.00 %) blocks
Max Waves: 28668 -> 27860 (-2.82 %)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>