[llvm] [ValueTracking] AllowEphemerals for alignment assumptions. (PR #108632)

[Nikita Popov via llvm-commits]

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

A reluctant LGTM. I think overall it's reasonable to ignore ephemerals for operand bundles by default and add a flag if/when we actively want to simplify them.

Always a good time to review our long tradition of "trying to fix the ephemeral value problem in SCEV":

https://reviews.llvm.org/D93974
https://reviews.llvm.org/D97077
https://reviews.llvm.org/D97092
https://reviews.llvm.org/D97099
https://reviews.llvm.org/D155389

I think https://reviews.llvm.org/D97092 has our latest understanding of this problem, and the discussion there at the end was leaning towards something along the lines of this PR.

https://github.com/llvm/llvm-project/pull/108632