[llvm] Add setBranchWeigths convenience function. NFC (PR #72446)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi approved this pull request.

LGTM. I was surprised that `Instruction` didn't implement this already, but I think this is a good change. Thanks for the nice improvement.

https://github.com/llvm/llvm-project/pull/72446