nir: Add a small pass to rematerialize derefs per-block

nir: Add a small pass to rematerialize derefs per-block

This pass re-materializes deref instructions on a per-block basis to
ensure that every use of a deref occurs in the same block as the
instruction which uses it.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Cc: "18.2" <mesa-stable@lists.freedesktop.org>