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>

src/compiler/nir/nir_lower_bitmap.c

@@ -128,12 +128,7 @@ void
 nir_lower_bitmap(nir_shader *shader,
                  const nir_lower_bitmap_options *options)
 {
-   nir_function *function;
-
    assert(shader->stage == MESA_SHADER_FRAGMENT);
 
-   function = nir_shader_get_entrypoint(shader);
-
-   if (function->impl)
-      lower_bitmap_impl(function->impl, options);
+   lower_bitmap_impl(nir_shader_get_entrypoint(shader), options);
 }