radeonsi: don't fetch 8 dwords for samplerBuffer and imageBuffer

radeonsi: don't fetch 8 dwords for samplerBuffer and imageBuffer

The compiler doesn't shrink s_load_dwordx8, so we always wasted 4 SGPRs.
Also, the extraction of the descriptor created some really ugly asm code
with lots of VALU bitwise ops and v_readfirstlane.

Totals from *affected* shaders:
SGPRS: 13880 -> 13253 (-4.52 %)
VGPRS: 15200 -> 15088 (-0.74 %)
Code Size: 499864 -> 459816 (-8.01 %) bytes
Max Waves: 1554 -> 1564 (0.64 %)

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>