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

[via llvm-commits]

ayalz 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.
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> This should be sufficient to unblock VPlan2VPlan interleaving.
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> WDYT?

Looks good to me, with various minor comments.

> of VPlan only: w/o delegating to names of IR Values when generating them,

Actually, it appears the (two listed VPInstruction) names do not even propagate to VPlan's debug printings, i.e., they are totally unused and should be dce'd.

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