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

[Thomas Symalla via llvm-commits]

tsymalla wrote:

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

Yep. I wanted to see if the change is generally accepted at first, before changing all the code. I'll follow up with the remainder.

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