[llvm] [SCEV] Don't add use flags already present on expression. (PR #219016)

[Nikita Popov via llvm-commits]

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

LGTM

Probably worth noting that we can still end up with redundant at-use flags if global flags get set on the SCEV after the fact.

https://github.com/llvm/llvm-project/pull/219016