inaccessiblememonly means that it doesn't modify memory accesible via
normal LLVM pointers. This lets LLVM's dead store elimination, memcpy
forwarding, etc. ignore functions with this attribute. We don't
represent descriptors as pointers, so this property is always true of
buffer and image stores. There are plans to represent descriptors via
pointers, but this just means that now nothing is inaccessiblememonly,
as LLVM will then understand loads/stores via its usual alias analysis.

Radeonsi was mistakenly only setting it if the driver could prove that
there were no reads, and then it was cargo-culted into ac_llvm_build
and ac_llvm_to_nir. Rip it out of everything.

statistics with nir enabled:

Totals from affected shaders:
SGPRS: 152 -> 152 (0.00 %)
VGPRS: 128 -> 132 (3.12 %)
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: 9324 -> 9244 (-0.86 %) bytes
LDS: 2 -> 2 (0.00 %) blocks
Max Waves: 17 -> 17 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

The only difference was a manhattan31 shader.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>