ac/nir: export some undef as zero

ac/nir: export some undef as zero

NIR already optimizes undef usage.
If undef reaches llvm, it's probably because of a broken shader.

In this situation, rather than letting llvm use the undef values
to do more optimization and probably produce incorrect results,
we replace undef values by 0.

"undef" values that are directly used in exports are kept as undef,
because this allows llvm to optimize them away.

This is only enabled for radeonsi.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2689
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4607>