[llvm] [DirectX] Prevent `dxil-resource-access` from constructing a foldable phi (PR #211346)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

I think it'd be quite a bit simpler to just construct the PHI nodes and then use `PHINode::hasConstantValue` to decide if we really need them and delete them if we need to. This does do a bit of extra memory traffic if it turns out we don't need the PHI, but it's probably not too bad.

https://github.com/llvm/llvm-project/pull/211346