spirv: Add a parent field to vtn_cf_node

Cherry-pick'd from 2de5a41595442a02b5375d13082e236e1475c0bf

This makes it easier to crawl up the CF tree when trying to validate the
incoming SPIR-V control-flow.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3820>
Change-Id: I5d1417eb6bdfcd4952c81144bbaffee0073ffe17
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/403196
Reviewed-by: John Bauman <jbauman@google.com>
2 files changed