nir: Add function nir_function_set_impl
his function is added for create strong relationship between nir_function_impl and nir_function.
So that nir_function->impl->function == nir_function is always true when (nir_function->impl != NULL && nir_function->impl != NIR_SERIALIZE_FUNC_HAS_IMPL)
This is the result of review code !23807 (merged)
When reading nir_foreach_function_impl
, I can not found the strong relationship between
nir_function and nir_function_impl, so created this MR
Signed-off-by: Yonggang Luo luoyonggang@gmail.com