nir/inline_functions: Break inlining into a builder helper

nir/inline_functions: Break inlining into a builder helper

This pulls the guts of function inlining into a builder helper so that
it can be used elsewhere.  The rest of the infrastructure is still
needed for most inlining cases to ensure that everything gets inlined
and only ever once.  However, there are use-cases where you just want to
inline one little thing.  This new helper also has a neat trick where it
can seamlessly inline a function from one nir_shader into another.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>