[VPlan] Handle VPExpandSCEVRecipe in isUniformAfterVectorization.

VPExpandSCEVRecipes must be placed in the entry and are alway uniform.
This fixes a crash by always identifying them as uniform, even if the
main vector loop region has been removed.

Fixes llvm#121897.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -49,7 +49,8 @@ inline bool isUniformAfterVectorization(const VPValue *VPV) {
     return all_of(GEP->operands(), isUniformAfterVectorization);
   if (auto *VPI = dyn_cast<VPInstruction>(Def))
     return VPI->isSingleScalar() || VPI->isVectorToScalar();
-  return false;
+  // VPExpandSCEVRecipes must be placed in the entry and are alway uniform.
+  return isa<VPExpandSCEVRecipe>(Def);
 }
 
 /// Return true if \p V is a header mask in \p Plan.