Move snode information outside "runtime" struct

[ljcc0930]

This would be useful for #2506

• [ ] root pointers https://github.com/taichi-dev/taichi/blob/cd96133e9e45df588c53042099a0b8b37b6c75e1/taichi/runtime/llvm/runtime.cpp#L529-L530
• [ ] elements https://github.com/taichi-dev/taichi/blob/cd96133e9e45df588c53042099a0b8b37b6c75e1/taichi/runtime/llvm/runtime.cpp#L534-L536

[bobcao3]

This can also help implementing a unified device API