nir: add vertex divergence into nir_divergence_analysis

This is a prerequisite for the new nir_opt_varyings pass.
It reuses the same divergent field in nir_def and nir_loop.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26918>
2 files changed