nir: Add Panfrost-specific blending intrinsic
This gives more flexibility than the normal store_deref/store_output versions (particularly, it allows us to abuse the type system in awful ways, which is necessary for efficient format conversion in blend shaders.)
Signed-off-by: Alyssa Rosenzweig alyssa.rosenzweig@collabora.com
See today's render target series on the ML (it's massive, you can't miss it) for the use.