[llvm] [DAGCombiner] Fix to avoid writing outside original store in ReduceLoadOpStoreWidth (PR #119203)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

On the SDAG level, I think it might be valid to introduce such an "out of bounds" access if we know the target won't trap, but it doesn't look like this code performs the checks that would be necessary to make that safe...

https://github.com/llvm/llvm-project/pull/119203