aco: increase accuracy of SGPR limits

aco: increase accuracy of SGPR limits

SGPRs are allocated in groups of 16 on GFX8/GFX9. GFX10 allocates a fixed
number of SGPRs and has 106 addressable SGPRs.

pipeline-db (Vega):
SGPRS: 5912 -> 6232 (5.41 %)
VGPRS: 1772 -> 1780 (0.45 %)
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: 88228 -> 87904 (-0.37 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 559 -> 571 (2.15 %)

piepline-db (Navi):
SGPRS: 341256 -> 363384 (6.48 %)
VGPRS: 171536 -> 170960 (-0.34 %)
Spilled SGPRs: 832 -> 581 (-30.17 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 14207332 -> 14190872 (-0.12 %) bytes
LDS: 33 -> 33 (0.00 %) blocks
Max Waves: 18072 -> 18251 (0.99 %)

v2: unconditionally count vcc as an extra sgpr on GFX10+
v3: pass SGPRs rounded to 8

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>