nir: Factor out nir_src_rewrite_ssa helper

nir: Factor out nir_src_rewrite_ssa helper

Like nir_instr_rewrite_ssa but without the asserted extra argument. Works on ifs
too, now that we have a unified use list.

We do need to assert that the source has actually been inserted and has valid
use/def chains. Previously, asserting on the parent instruction accomplished
that indirectly. For the more general helper, we instead directly assert that
there exists a non-null parent, whatever it is.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22343>