nir: properly find the entry to keep in copy_prop_vars

nir: properly find the entry to keep in copy_prop_vars

When copy propagation handles a store/copy, it iterates the current
copy entries to remove aliases, but keeps the "equal" entry (if
exists) to be updated.

The removal step may swap the entries around (to ensure there are no
holes), invalidating previous iteration pointers.  The bug was saving
such pointer to use later.  Change the code to first perform the
removals and then find the remaining right entry.

This was causing updates to be lost since they were being made to an
entry that was not part of the current copies.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108624
Fixes: b3c61469255 "nir: Copy propagation between blocks"
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>