There's no guarantee that we don't have more than 128 PHI values either,
so let's stop asuming so.
We do this by changing dxil_phi_set_incoming to dxil_phi_add_incoming,
which lets us add more incoming phi-values to the current one instead of
setting a new set of them.
This also lets us reduce these stack-arrays a bit, down to something
much more reasonable.
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15519>
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