[llvm] [VPlan] Add initial VPScalarEvolution, use to get trip count SCEV (NFC) (PR #94464)

[via llvm-commits]

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@@ -3477,8 +3474,8 @@ class VPlan {
   /// middle VPBasicBlock. If a check is needed to guard executing the scalar
   /// epilogue loop, it will be added to the middle block, together with
   /// VPBasicBlocks for the scalar preheader and exit blocks.
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ayalz wrote:

nit: would be good to explain what `IdxTy` stands for, and/or use a more descriptive name.

https://github.com/llvm/llvm-project/pull/94464