[compiler] Remove a redundant ToNumber conversion.

R=jarin@chromium.org

Bug: 
Change-Id: Ia674f7745ea08b31f3e3098c2d09bd05afc59ea7
Reviewed-on: https://chromium-review.googlesource.com/768410Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49359}

src/compiler/js-typed-lowering.cc

@@ -430,7 +430,6 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
   JSBinopReduction r(this, node);
   if (r.BothInputsAre(Type::Number())) {
     // JSAdd(x:number, y:number) => NumberAdd(x, y)
-    r.ConvertInputsToNumber();
     return r.ChangeToPureOperator(simplified()->NumberAdd(), Type::Number());
   }
   if (r.BothInputsAre(Type::PlainPrimitive()) &&