panfrost: Promote blend shader outputs 8->16-bit

..on Bifrost and later, where the conversion hardware makes this reasonable.
This saves us from inserting a pile of conversions in the compiler to lower away
the 8-bit input/output. This also generates substantially better code.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17841>
2 files changed