[llvm] [RemoveDIs] Replicate dbg intrinsic movement pattern in SelectOptimize (PR #81737)

[Jeremy Morse via llvm-commits]

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

LGTM, and this matches a few similar scenarios that I've seen before.

I think this makes the fifth or sixth place in LLVM where we have to manually call `setHeadBit`, which suggests we need to add some kind of `getAfterIterator()` method or some other way of producing a position with the head bit set. Otherwise it'll be harder for developers reading this to understand what's going on.

https://github.com/llvm/llvm-project/pull/81737