spirv: Only emit functions which are actually used

spirv: Only emit functions which are actually used

Instead of emitting absolutely everything, just emit the few functions
that are actually referenced in some way by the entrypoint.  This should
save us quite a bit of time when handed large shader modules containing
many entrypoints.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>