[llvm] [PatternMatch] Allow poison in api_pred_ty matchers (PR #89188)

[John Regehr via llvm-commits]

regehr wrote:

> maybe @regehr can help with fuzzing vector instructions.

yes, we have a mutation-based fuzzer for IR. currently I'm using it on my biggest machine to look for AArch64 backend bugs, but when this run finishes, I can turn the tools back towards middle-end bugs. it's easy to filter out inputs that don't contain any vectors. I'll do that next.

https://github.com/llvm/llvm-project/pull/89188