llvmpipe nir support - initial attempt
I've had an MR open for this !2303 (merged) but the commits got messy. So I'm going to try and get a clean initial off by default NIR port for llvmpipe.
This set of commits should be more reviewable, there are a number of regressions from TGSI to NIR but I intend to use follow up MRs to fix those in easier to review chunks.