[VPlan] Assert instead of check if VF is vector when widening GEPs(NFC)

VPWidenGEPRecipe should not be generated for scalar VFs. Replace
check with an assert.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -787,6 +787,7 @@ void VPScalarIVStepsRecipe::print(raw_ostream &O, const Twine &Indent,
 #endif
 
 void VPWidenGEPRecipe::execute(VPTransformState &State) {
+  assert(State.VF.isVector() && "not widening");
   auto *GEP = cast<GetElementPtrInst>(getUnderlyingInstr());
   // Construct a vector GEP by widening the operands of the scalar GEP as
   // necessary. We mark the vector GEP 'inbounds' if appropriate. A GEP
@@ -794,7 +795,7 @@ void VPWidenGEPRecipe::execute(VPTransformState &State) {
   // is vector-typed. Thus, to keep the representation compact, we only use
   // vector-typed operands for loop-varying values.
 
-  if (State.VF.isVector() && areAllOperandsInvariant()) {
+  if (areAllOperandsInvariant()) {
     // If we are vectorizing, but the GEP has only loop-invariant operands,
     // the GEP we build (by only using vector-typed operands for
     // loop-varying values) would be a scalar pointer. Thus, to ensure we