[llvm] [DropUnnecessaryAssumes] Make the ephemeral value check more precise (PR #160700)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

> Interestingly, this doesn't seem to actually matter in practice (no difference on llvm-opt-benchmark).

Not sure how I feel about this. I've personally abandoned patches in the past due to zero-diff (or even small diff) on llvm-opt-benchmark. The patch is certainly nice for completeness though?

https://github.com/llvm/llvm-project/pull/160700