[llvm] [VPlan] Model initial header mask as region value. (PR #196199)

[Luke Lau via llvm-commits]

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@@ -171,13 +134,6 @@ const SCEV *vputils::getSCEVExprForVPValue(const VPValue *V,
                                            PredicatedScalarEvolution &PSE,
                                            const Loop *L) {
   ScalarEvolution &SE = *PSE.getSE();
-  if (isa<VPIRValue, VPSymbolicValue>(V)) {
-    Value *LiveIn = V->getUnderlyingValue();
-    if (LiveIn && SE.isSCEVable(LiveIn->getType()))
-      return SE.getSCEV(LiveIn);
-    return SE.getCouldNotCompute();
-  }
-
   if (auto *RV = dyn_cast<VPRegionValue>(V)) {
     assert(RV == RV->getDefiningRegion()->getCanonicalIV() &&
            "RegionValue must be canonical IV");
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lukel97 wrote:

Can anything call getSCEVExprForVPValue with the header mask? Should this assert be weakened and just return getCouldNotCompute for non-canonicalIV VPRegionValues now?

https://github.com/llvm/llvm-project/pull/196199