pan/bi: Implement load_const

pan/bi: Implement load_const

In the laziest possible way...  We can just emit worst case moves which
DCE will eat for breakfast anyway, and inline constants on instructions
where that is supported directly. This approach eliminates a lot of
nasty corner cases that Midgard's crazy cache scheme has hit, at the
expense of slightly more work for DCE (but it's only a single iteration
of an O(N) pass that has to run anyway..)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4097>