[llvm] [LoopLoadElim] Fix miscompile caused by incorrect store-to-load forwarding across mixed-width clobbers (PR #224833)

[Hadong Lee via llvm-commits]

ChrisLee02 wrote:

> This change looks very complicated for the issue it fixes. Why is it not good enough to add the load to LoadsWithUnknownDependence?

I chose candidate-specific handling to avoid blocking valid forwarding when the mixed-width store is overwritten by the forwarding source.
The `overwritten_clobber` test case in mixed-width-clobber.ll covers this case.

https://github.com/llvm/llvm-project/pull/224833