nir/opt_vectorize: Add a callback for filtering of vectorizing.

nir/opt_vectorize: Add a callback for filtering of vectorizing.

For NIR-to-TGSI, we don't want to revectorize 64-bit ops that we split to
scalar beyond vec2 width.  We even have some ops that we would rather
retain as scalar due to TGSI opcodes being scalar, or having more unusual
requirements.

This could be used to do the vectorize_vec2_16bit filtering, but that
shader compiler option is also used in algebraic so leave it in place for
now.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6567>