lima/ppir: introduce ppir_node src accessors and store ppir_node pointer in ppir_src
- Introduce ppir_node src accessors to avoid duplicating this code in different places
- Utilize new accessors in regalloc
- Add ppir_node pointer into ppir_src (pointer is NULL if src is a reg)
- Print sources and destinations in ppir_node_print_prog() to catch bugs in ppir compiler when srcs or dests aren't updated when necessary.