[VPlan] Get Addr computation cost with scalar type if it is uniform for gather/scatter.

This patch query `getAddressComputationCost()` with scalar type if the
address is uniform. This can help the cost for gather/scatter more
accurate.

In current LV, non consecutive VPWidenMemoryRecipe (gather/scatter) will
account the cost of address computation. But there are some cases that
the addr is uniform accross lanes, that makes the address can be
calculated with scalar type and broadcast.

I have a follow optimization that try to converts gather/scatter with
uniform memory acces to scalar load/store + broadcast. With this
optimization, we can remove this temporary change.

Author: ElvisWang123

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3182,10 +3182,17 @@ InstructionCost VPWidenMemoryRecipe::computeCost(ElementCount VF,
     // TODO: Using the original IR may not be accurate.
     // Currently, ARM will use the underlying IR to calculate gather/scatter
     // instruction cost.
-    const Value *Ptr = getLoadStorePointerOperand(&Ingredient);
-    Type *PtrTy = toVectorTy(Ptr->getType(), VF);
     assert(!Reverse &&
            "Inconsecutive memory access should not have the order.");
+
+    const Value *Ptr = getLoadStorePointerOperand(&Ingredient);
+    Type *PtrTy = Ptr->getType();
+
+    // If the address value is uniform across all lanes, then the address can be
+    // calculated with scalar type and broadcast.
+    if (!vputils::isSingleScalar(getAddr()))
+      PtrTy = toVectorTy(PtrTy, VF);
+
     return Ctx.TTI.getAddressComputationCost(PtrTy, nullptr, nullptr,
                                              Ctx.CostKind) +
            Ctx.TTI.getGatherScatterOpCost(Opcode, Ty, Ptr, IsMasked, Alignment,