[llvm] [VPlan] Preserve underlying value when EVL transforming intrinsics/casts (PR #121552)

[Luke Lau via llvm-commits]

lukel97 wrote:

Closing for now to take off people's review queue, I'll reopen it once #118638 and #115274 land which should expose the problem

https://github.com/llvm/llvm-project/pull/121552