[llvm] [VPlan] Use RPOT in CSE, fixing potential crash (PR #187548)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I presume we can't add a test case until #187589, but can you verify that it crashes without this PR, and it's fixed with this PR?

Already verified, and there is a test in tree that should crash with #187589 without this patch.

https://github.com/llvm/llvm-project/pull/187548