nir: Change nir_shader_get_entrypoint to return an impl.

nir: Change nir_shader_get_entrypoint to return an impl.

Jason suggested adding an assert(function->impl) here.  All callers
of this function actually want ->impl, so I decided just to change
the API.

We also change the nir_lower_io_to_temporaries API here.  All but one
caller passed nir_shader_get_entrypoint(), and with the previous commit,
it now uses a nir_function_impl internally.  Folding this change in
avoids the need to change it and change it back.

v2: Fix one call I missed in ir3_compiler (caught by Eric).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>