[llvm] [GVN] Support rnflow pattern matching and transform (PR #162259)

[Alina Sbirlea via llvm-commits]

alinas wrote:

> IIUC, are you suggesting that we land this commit with off by default and then in the successive commit we just flip the switch to make the revert easy, in case of miscompiles?
Yes, precisely. If an issue is found, rather than reverting all changes in this PR, you can then revert the flag flip, fix the issue, then re-enable the flag.

https://github.com/llvm/llvm-project/pull/162259