nir/loop_analyze: remove cost of redundant selects

nir/loop_analyze: remove cost of redundant selects

If we know that a select will be eliminated once the loop is
unrolled than we don't need to count the instruction towards the
cost of the loop.

This change helps 2 loops unroll in an xcom enemy unknown shader
that is loaded full of these redundant selects.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18587>