[llvm] [VPlan] Re-use VPSlotTracker when printing recipes for costs (NFC). (PR #203386)

[Florian Hahn via llvm-commits]

fhahn wrote:

> @fhahn It seems the regression is because std::make_unique(...) is called unconditionally at VPCostContext call sites, while the tracker is only stored/used under #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP), so release builds construct (walking the whole VPlan to number all VPValues) and immediately throw away a slot tracker on every VPCostContext.

Yep, I am planning to re-land with lazily constructing the slot tracker, but only after the release branch has been cut

https://github.com/llvm/llvm-project/pull/203386