[llvm] [llubi] Extract reusable methods of InstExecutor into ExecutorAPI (PR #186976)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Mostly just nits from me, I think this is a reasonable approach.

I was originally thinking we could get away without refactoring this and just call `reportImmediateUB` for libcall arguments that end up being `poison` directly inside `callLibFunc`, but we also need to load memory for pointer arguments, so it probably makes sense to just refactor this out.

https://github.com/llvm/llvm-project/pull/186976