nir/lower_goto_if: Clean up ralloc usage

nir/lower_goto_if: Clean up ralloc usage

It's really hard to track in this pass which sets are getting ralloc'd
off which other sets.  To avoid leaks, just pass a mem_ctx around
everywhere and ralloc almost everything off the one context.  We do keep
using recursion a few places where it's crystal clear what the parent
relationship is.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>