This adds a NIR pass that decides which portions of UBOS we should
upload as push constants, rather than pull constants.
v2: Switch to uint16_t for the UBO block number, because we may
have a lot of them in Vulkan (suggested by Jason). Add more
comments about bitfield trickery (requested by Matt).
v3: Skip vec4 stages for now...I haven't finished wiring up support
in the vec4 backend, and so pushing the data but not using it
will just be wasteful.
Reviewed-by: Matt Turner <mattst88@gmail.com>
