[llvm] [GVN] Improve processBlock for instruction erasure (PR #131753)

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

@antoniofrighetto @nikic Addressed all review comments. The thing about guarding assert under `V != nullptr` in `verifyRemoved()` was added for some test but I don't remember which one. Running `ninja check-all` today leads to no new failures, thus I have removed that change. Probably the issue is fixed somewhere else.

https://github.com/llvm/llvm-project/pull/131753