[llvm] [VPlan] Track VPValue names in VPlan. (PR #81411)

[Florian Hahn via llvm-commits]

fhahn wrote:

I completely reworked the patch to now *only* perform de-duplication in `VPSlotTracker`, without changing anything else besides `printAsOperand` to use `VPSlotTracker::getName` to obtain a possibly de-duplicated name for a VPValue.

This should be sufficient to unblock VPlan2VPlan interleaving

https://github.com/llvm/llvm-project/pull/81411