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>