[llvm] [InstCombine] Forward memcpy source to load instruction (PR #140249)

[Nikita Popov via llvm-commits]

nikic wrote:

The general guideline is that InstCombine load forwarding should be a subset of GVN load forwarding. New capabilities should generally be added to GVN/MDA first to handle the most generic case (including the cross-block one above). Support in InstCombine exists to avoid some phase ordering issues.

(What I have in mind here is not to replace the load from dst with a load from src, but with the available value of the load from src if one exists. But this is probably not easy, because we don't really have a good way to handle pointer offsets that are not materialized in IR.)

https://github.com/llvm/llvm-project/pull/140249