[llvm] [IR] Implement successors as Use iterators (PR #186616)

[Alexis Engelke via llvm-commits]

aengelke wrote:

[stage2-O3 -0.26%](http://llvm-compile-time-tracker.com/compare.php?from=4051be60d6dda2a43178ef97103ec1dab6850320&to=4b5414c395bafcafc49a3ea8c9ad37d16005c834&stat=instructions:u)

https://github.com/llvm/llvm-project/pull/186616