nir: Do not use continue block after removing it.

Reinserting code directly before a jump means the block gets split
and merged, removing the original block and replacing it in the
process.

Hence keeping a pointer to the continue block over a reinsert
causes issues.

This code changes nir_opt_if to simply look for the new continue
block.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107275
CC: 18.1 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 8cacf38f)