[llvm] [RDA] Fix getGlobalUses() to account for redefinitions (PR #78178)

[Sam Parker via llvm-commits]

sparker-arm wrote:

Yes, that's exactly what I think we need. LowOverheadLoops uses RDA is a very specific way, so testing the analysis in isolation would be great. I'm just not aware of these kind of tests existing in LLVM, but I'm a bit of touch these days. 

https://github.com/llvm/llvm-project/pull/78178