radeonsi: get rid of constant buffer preloading

radeonsi: get rid of constant buffer preloading

26011 shaders in 14651 tests
Totals:
SGPRS: 1152636 -> 1146340 (-0.55 %)
VGPRS: 728198 -> 727371 (-0.11 %)
Spilled SGPRs: 3776 -> 2218 (-41.26 %)
Spilled VGPRs: 369 -> 369 (0.00 %)
Scratch VGPRs: 1344 -> 1344 (0.00 %) dwords per thread
Code Size: 35835152 -> 35841268 (0.02 %) bytes
LDS: 767 -> 767 (0.00 %) blocks
Max Waves: 222372 -> 222559 (0.08 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>