ac/nir: begin splitting off ac_nir_context

ac/nir: begin splitting off ac_nir_context

The eventual goal is to hide all radv-specific details behind
ac_nir_context::abi, so that the NIR->LLVM code can be re-used by
radeonsi.

During development, we live with a partial split, where some of the
NIR->LLVM code still relies on linking back to the nir_to_llvm_context
(which should ultimately be renamed to reflect that it's radv-specific).
The idea is to get rid of these backlinks over time.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>