[llvm-branch-commits] [llvm] release/20.x: Do not treat llvm.fake.use as a debug instruction (#128684) (PR #128734)

[Orlando Cazalet-Hyams via llvm-branch-commits]

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

LGTM - IIUC without this patch we run into failed verifier checks, e.g. in the SimplifyCFG case outlined in the test.

This patch could have a knock-on effect, blocking optimisations beyond the one in the test, but that's what the user signs up for with the `-fextend-variable-liveness` flag, so I think this is ok. Definitely better to do the right thing (this) now (in this release), and tweak specific optimisations later on a case by case basis, if we discover better trade offs.

https://github.com/llvm/llvm-project/pull/128734