Revert "[turbofan] Consistently use String feedback for JSAdd."

This reverts commit d504203e.

Reason for revert: b/74469145

Original change's description:
> [turbofan] Consistently use String feedback for JSAdd.
> 
> Currently we didn't always consistently use the String feedback on
> JSAdd, but only if JSTypedLowering would already figure out statically
> that one of the inputs is already a String. That leads to some odd
> performance cliffs, as highlighted in the referenced bug.
> 
> This CL fixes the JSTypedLowering::ReduceJSAdd to always bake in the
> String feedback. This improves the relevant performance tests from the
> bug from
> 
>   console.timeEnd: Runtime join3, 967.512000
>   console.timeEnd: Runtime join, 1004.599000
>   console.timeEnd: Runtime join3, 1124.764000
>   console.timeEnd: Runtime join, 966.164000
>   console.timeEnd: Runtime join3, 1145.296000
>   console.timeEnd: Runtime join, 966.176000
>   console.timeEnd: Runtime join3, 1145.272000
>   console.timeEnd: Runtime join, 931.266000
> 
> to
> 
>   console.timeEnd: Runtime join3, 903.050000
>   console.timeEnd: Runtime join, 856.509000
>   console.timeEnd: Runtime join3, 945.144000
>   console.timeEnd: Runtime join, 840.038000
>   console.timeEnd: Runtime join3, 927.965000
>   console.timeEnd: Runtime join, 841.263000
>   console.timeEnd: Runtime join3, 929.342000
>   console.timeEnd: Runtime join, 858.143000
> 
> which corresponds to an 8-18% improvement.
> 
> Bug: v8:7415
> Change-Id: I62e008298e4ee0864885b37817c91d055acf2a09
> Reviewed-on: https://chromium-review.googlesource.com/936643
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51551}

TBR=jarin@chromium.org,bmeurer@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7415
Change-Id: I106a314bcd4187abdad6dc11306226d0c28ef524
Reviewed-on: https://chromium-review.googlesource.com/963522Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51949}

src/compiler/js-typed-lowering.cc

@@ -504,13 +504,6 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
     r.ConvertInputsToNumber();
     return r.ChangeToPureOperator(simplified()->NumberAdd(), Type::Number());
   }
-  if (BinaryOperationHintOf(node->op()) == BinaryOperationHint::kString) {
-    // Always bake in String feedback into the graph.
-    // TODO(bmeurer): Consider adding a SpeculativeStringAdd operator,
-    // and use that in JSTypeHintLowering instead of looking at the
-    // binary operation feedback here.
-    r.CheckInputsToString();
-  }
   if (r.OneInputIs(Type::String())) {
     // We know that (at least) one input is already a String,
     // so try to strength-reduce the non-String input.
@@ -546,7 +539,7 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
       return ReduceCreateConsString(node);
     }
     // Eliminate useless concatenation of empty string.
-    if (r.BothInputsAre(Type::String())) {
+    if (BinaryOperationHintOf(node->op()) == BinaryOperationHint::kString) {
       Node* effect = NodeProperties::GetEffectInput(node);
       Node* control = NodeProperties::GetControlInput(node);
       if (r.LeftInputIs(empty_string_type_)) {
@@ -589,8 +582,6 @@ Reduction JSTypedLowering::ReduceJSAdd(Node* node) {
     NodeProperties::ChangeOp(node, common()->Call(call_descriptor));
     return Changed(node);
   }
-  // We never get here when we had String feedback.
-  DCHECK_NE(BinaryOperationHint::kString, BinaryOperationHintOf(node->op()));
   return NoChange();
 }