[llvm] [IR][Instructions] Add `CallBase::getCalledFunctionName` helper (PR #127038)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

>From a quick git grep "getCalledFunction()->getName()", it looks like there are a lot more places that could use this.

https://github.com/llvm/llvm-project/pull/127038