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

[via llvm-commits]

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@@ -45,6 +50,11 @@ inline bool isUniformAfterVectorization(const VPValue *VPV) {
 
 /// Return true if \p V is a header mask in \p Plan.
 bool isHeaderMask(const VPValue *V, VPlan &Plan);
+
+/// Return the SCEV expression for \p V. Returns SCEVCouldNotCompute if no
+/// SCEV expression could be constructed.
+const SCEV *getSCEVExprForVPValue(VPValue *V, ScalarEvolution &SE);
+
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ayalz wrote:

This complements `getOrCreateVPValueForSCEVExpr()` above and should be placed next to it.
(Otherwise would have suffice to call it getSCEVFor() or getSCEV(), as it currently delegates to SE.getSCEV() or SCEVExpandRecioe.getSCEV(). Could even have `getOrCreateSCEVExprForVPValue()` more consistently, but getSCEV() may typically create them - in contrast to getExistingSCEV().)

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