nv50/ir/tgsi: handle TGSI_OPCODE_LOAD,STORE

nv50/ir/tgsi: handle TGSI_OPCODE_LOAD,STORE

Squashed and (heavily) modified original patches by Francisco Jerez:
nv50/ir/tgsi: Implement resource LOAD/STORE (wip).
nv50/ir/tgsi: Emit SUST/SULD for surface access, and add CB LOAD/STORE support
nv50/ir/tgsi: Fix/clean up the LOAD/STORE handling code.

Left out for now:
nv50/ir/tgsi: Resource indirect indexing

Treating raw, read-only surfaces as constant buffers (CBs) was removed
because CBs are limited to a size of 64 KiB which isn't desireable, and
because this decision should probably be made by the state tracker.
If we used a number of CB slots for surfaces, it might find that we
cannot accomodate the advertised limit.