[llvm] [verify][swift] Allow passing swifterror to phi instructions (PR #138598)

[Ellis Hoag via llvm-commits]

ellishg wrote:

> Any background on why people haven't previously encountered PHI nodes here?

I'm not sure, but I can shed some light on how I encountered it. I suspect that most people compiling Swift code do not run the verifier.

It turns out that `llvm::UpgradeDebugInfo()` always runs `verifyModule()`, which eventually hits this failure. In https://github.com/llvm/llvm-project/pull/132115 I tried to disable this with a flag, but ended up closing the PR. Luckily the fix wasn't so bad.

https://github.com/llvm/llvm-project/pull/138598