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

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto commented:

This looks good to me, though I also wonder whether it would make sense to give a try to just enable verifyRemoved() under EXPENSIVE_CHECKS, as per previous @nikic suggestion.

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