[turbofan] Eliminate redundant CheckString based on types.

If the input to a CheckString is already a String, then the check
is redundant.

R=epertoso@chromium.org

Review-Url: https://codereview.chromium.org/2190833002
Cr-Commit-Position: refs/heads/master@{#38118}

src/compiler/js-typed-lowering.cc

@@ -1889,6 +1889,17 @@ Reduction JSTypedLowering::ReduceSelect(Node* node) {
   return NoChange();
 }
 
+Reduction JSTypedLowering::ReduceCheckString(Node* node) {
+  // TODO(bmeurer): Find a better home for this thing!
+  Node* const input = NodeProperties::GetValueInput(node, 0);
+  Type* const input_type = NodeProperties::GetType(input);
+  if (input_type->Is(Type::String())) {
+    ReplaceWithValue(node, input);
+    return Replace(input);
+  }
+  return NoChange();
+}
+
 Reduction JSTypedLowering::ReduceNumberRoundop(Node* node) {
   // TODO(bmeurer): Find a better home for this thing!
   Node* const input = NodeProperties::GetValueInput(node, 0);
@@ -2019,6 +2030,8 @@ Reduction JSTypedLowering::Reduce(Node* node) {
       return ReduceJSGeneratorRestoreRegister(node);
     case IrOpcode::kSelect:
       return ReduceSelect(node);
+    case IrOpcode::kCheckString:
+      return ReduceCheckString(node);
     case IrOpcode::kNumberCeil:
     case IrOpcode::kNumberFloor:
     case IrOpcode::kNumberRound:

src/compiler/js-typed-lowering.h

@@ -78,6 +78,7 @@ class JSTypedLowering final : public AdvancedReducer {
   Reduction ReduceJSGeneratorStore(Node* node);
   Reduction ReduceJSGeneratorRestoreContinuation(Node* node);
   Reduction ReduceJSGeneratorRestoreRegister(Node* node);
+  Reduction ReduceCheckString(Node* node);
   Reduction ReduceNumberRoundop(Node* node);
   Reduction ReduceSelect(Node* node);
   Reduction ReduceJSSubtract(Node* node);