[llvm] [DebugInfo] Update policy for when to merge locations (PR #115349)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

I've added an example for multiple merges - it's not something that actually happens in the compiler right now, but primarily I think because DILocations aren't handled correctly through a lot of vectorizer operations (vector adds seem to just take a location from one of the input adds, for example). There is an actual example in `AMDGPULibCalls::fold_sincos`, where we create several instructions to replace separate `sin` and `cos` instructions with a merged `sincos` call, but the logic is a little obtuse for a small example imo.

https://github.com/llvm/llvm-project/pull/115349