pan/bi: Register allocate BLEND dest on Valhall

pan/bi: Register allocate BLEND dest on Valhall

On Bifrost, BLEND writes to the link register, acting like a function call. On
Valhall, BLEND does not write anything. But the BLEND instruction in our IR is a
pseudo-instruction with Bifrost semantics, expanding to a multi-instruction
sequence on Valhall. So it's not worth special casing Valhall in instruction
selection.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>