WIP: spirv, i965: use variables to handle SSBO/UBOs for ARB_gl_spirv
This currently MR is on top of !178 (merged).
Created the MR to make it easier to view and comment on the changes. If this approach is accepted we may decide whether to keep those as a MR on top of !178 (merged) or if I just mix the patches into that MR (and remove the ones not needed), so it gets absorbed by it.