[llvm] [AggressiveInstCombine] Guard memset with length in [0, 1] (PR #213240)

[via llvm-commits]

hsnaveen2u wrote:

@antoniofrighetto 
Thanks for the review.

The patch has been updated to address all three points:
- The branch has been rebased onto current main after #217224 landed.
- Title updated to [AggressiveInstCombine] Guard memset with length in [0, 1]; the testing bullet list has been removed.
- The transform is now in a dedicated foldMemSetZeroOrOneLength(MemSetInst *, ...) function following the same pattern as foldLibCalls.

https://github.com/llvm/llvm-project/pull/213240