[llvm] [EarlyCSE] Compare GEP instructions based on offset (PR #65875)

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DianQK wrote:

Using references works well!
https://llvm-compile-time-tracker.com/compare.php?from=ac1daad9bb4eb083df6b215c029816d3149e00d8&to=089349f9eb47c57cfa2049ce0af5c7dd079a8581&stat=instructions%3Au

I've also tried some approaches. 
It should be useful to avoid offset calculations. I tried counting only with a GEP i8, but there is some extra cost to the determination of this one.
Simplifying the offset calculation is of some use, but I'm not sure it makes sense. Results are here https://llvm-compile-time-tracker.com/compare.php?from=ac1daad9bb4eb083df6b215c029816d3149e00d8&to=8ba478eb77c6dc09b87e9b511614e45e167be2b5&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/65875