tu: Refactor shader compilation flow

tu: Refactor shader compilation flow

In order to do cross-stage linking, we'll need to split out SPIR-V->NIR
and NIR finalization, so that we can do a round of linking in between.
The multiview lowering pass also assumes that it sits between two
optimization loops, which in anv are the pre-linking optimizations and
post-linking finalization.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6515>