ac/nir: rewrite shared variable handling (v2)

ac/nir: rewrite shared variable handling (v2)

Translate the NIR variables directly to LLVM instead of lowering to a
TGSI-style giant array of vec4's and then back to a variable. This
should fix indirect dereferences, make shared variables more tightly
packed, and make LLVM's alias analysis more precise. This should fix an
upcoming Feral title, which has a compute shader that was failing to
compile because the extra padding made us run out of LDS space.

v2: Combine the previous two patches into one, only use this for shared
variables for now until LLVM becomes smarter.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Alex Smith <asmith@feralinteractive.com>