[VPlan] Handle SELECT in isSingleScalar.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -49,6 +49,7 @@ inline bool isSingleScalar(const VPValue *VPV) {
     case Instruction::GetElementPtr:
     case Instruction::ICmp:
     case Instruction::FCmp:
+    case Instruction::Select:
     case VPInstruction::Broadcast:
     case VPInstruction::PtrAdd:
       return true;