Revert of [crankshaft] Re-add fast-case for string add left/right.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HYDROGEN_INSTRUCTIONS_H_
 #define V8_HYDROGEN_INSTRUCTIONS_H_
 
 #include <cstring>
 #include <iosfwd>
 
@@ skipping 7337 matching lines @@
   Unique<Map> original_map_;
   Unique<Map> transitioned_map_;
   uint32_t bit_field_;
 };
 
 
 class HStringAdd final : public HBinaryOperation {
  public:
   static HInstruction* New(
       Isolate* isolate, Zone* zone, HValue* context, HValue* left,
-      HValue* right, PretenureFlag pretenure_flag = NOT_TENURED,
+      HValue* right, Strength strength = Strength::WEAK,
+      PretenureFlag pretenure_flag = NOT_TENURED,
       StringAddFlags flags = STRING_ADD_CHECK_BOTH,
       Handle<AllocationSite> allocation_site = Handle<AllocationSite>::null());
 
   StringAddFlags flags() const { return flags_; }
   PretenureFlag pretenure_flag() const { return pretenure_flag_; }
 
   Representation RequiredInputRepresentation(int index) override {
     return Representation::Tagged();
   }
 
   std::ostream& PrintDataTo(std::ostream& os) const override;  // NOLINT
 
   DECLARE_CONCRETE_INSTRUCTION(StringAdd)
 
  protected:
   bool DataEquals(HValue* other) override {
     return flags_ == HStringAdd::cast(other)->flags_ &&
         pretenure_flag_ == HStringAdd::cast(other)->pretenure_flag_;
   }
 
  private:
-  HStringAdd(HValue* context, HValue* left, HValue* right,
+  HStringAdd(HValue* context, HValue* left, HValue* right, Strength strength,
              PretenureFlag pretenure_flag, StringAddFlags flags,
              Handle<AllocationSite> allocation_site)
-      : HBinaryOperation(context, left, right, Strength::WEAK, HType::String()),
+      : HBinaryOperation(context, left, right, strength, HType::String()),
         flags_(flags),
         pretenure_flag_(pretenure_flag) {
     set_representation(Representation::Tagged());
     SetFlag(kUseGVN);
     SetDependsOnFlag(kMaps);
     SetChangesFlag(kNewSpacePromotion);
     if (FLAG_trace_pretenuring) {
       PrintF("HStringAdd with AllocationSite %p %s\n",
              allocation_site.is_null()
                  ? static_cast<void*>(NULL)
@@ skipping 581 matching lines @@
 };
 
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_