[llvm] [GVN][PGO] Skip GVN if entry BlockFreq is 0 (PR #166336)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

> The real culprit is the MD algorithms, which easily achieve quadratic complexity. I have been pushing for MSSA migration, but we have had little progress over the last few weeks. Once we migrate to MSSA, I expect the issue to go away.

This is a MSSA migration for an upstream pass or for an internal migration?

Code formatting needs fixing before landing, but otherwise LGTM. It would be good if you can commit to removing this flag once the motivating use case for this is resolved properly. Not sure if there's anyone else who would be good for reviewing GVN patches. Nikita is out until December. I think this is probably fine to land as is.

https://github.com/llvm/llvm-project/pull/166336