[llvm] [GVN] use `AssertingVH` for leaders to improve compilation time (PR #175870)

[Princeton Ferro via llvm-commits]

Prince781 wrote:

> Is it possible to add lit test that exercise the path in GVN?

We don't currently have compile-time timeout tests, which is what would be needed here. The existing GVN tests should be good enough coverage for this change.

https://github.com/llvm/llvm-project/pull/175870