radv,ac/nir: lower deref operations for shared memory

radv,ac/nir: lower deref operations for shared memory

This shouldn't introduce any functional changes for RadeonSI
when NIR is enabled because these operations are already lowered.

pipeline-db (NAVI10/LLVM):
SGPRS: 9043 -> 9051 (0.09 %)
VGPRS: 7272 -> 7292 (0.28 %)
Code Size: 638892 -> 621628 (-2.70 %) bytes
LDS: 1333 -> 1331 (-0.15 %) blocks
Max Waves: 1614 -> 1608 (-0.37 %)

Found this while glancing at some F12019 shaders.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>