nir/opt_if: Clean up single-src phis in opt_if_loop_terminator

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111071


Fixes: 2a74296f "nir: add opt_if_loop_terminator()"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 7a19e05e)