[llvm] [VPlan] Fix mutating whilst iterating over users in EVL transform (PR #122885)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

> Thanks!:) I will try this patch tomorrow to see if it fixes the remaining fails in llvm test-suite.

I confirmed the remaining 4 fails in llvm test-suite are fixed by this patch. 

https://github.com/llvm/llvm-project/pull/122885