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

[via llvm-commits]

LiqinWeng wrote:

I'm more curious about how you extracted miscompilation extracted from 525.x264_r,. Can you teach me?:)（
I ran the spec2017 of llvm-test-suit but there was no prompt information

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