[llvm] [mlir] [clang] [llvm] Improve implementation of StringRef::find_last_of and cie (PR #71865)

[Kazu Hirata via cfe-commits]

https://github.com/kazutakahirata commented:

This patch looks good to me.  I am not sure if we want to implement architecture-specific optimizations like `__SSE2__` at the source-code level.  But then I don't want that to block the landing of this patch.  Is there any way you could separate these two patches?  Thanks!

https://github.com/llvm/llvm-project/pull/71865