[llvm] [StackProtector] Fix phi handling in HasAddressTaken() (PR #129248)

[John Brawn via llvm-commits]

https://github.com/john-brawn-arm approved this pull request.

I've gotten most of the way through implementing my idea to check that it works, so I think that it makes sense to approve this so you can merge it (because it does fix this bug) and then I'll push a patch for review.

https://github.com/llvm/llvm-project/pull/129248