[llvm] [IRBuilder] Add CreatePtrAdd() method (NFC) (PR #77582)

[Nikita Popov via llvm-commits]

nikic wrote:

@dtcxzyw Thanks, I've switched more places to use CreatePtrAdd. I left a handful of places alone, because I felt that the i8 was semantically meaningful.

https://github.com/llvm/llvm-project/pull/77582