[compiler] Introduce a simple store-store elimination, disabled by default.

src/machine-type.h

 // Copyright 2014 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_MACHINE_TYPE_H_
 #define V8_MACHINE_TYPE_H_
 
 #include <iosfwd>
 
 #include "src/base/bits.h"
@@ skipping 48 matching lines @@
   MachineSemantic semantic() const { return semantic_; }
 
   bool IsSigned() {
     return semantic() == MachineSemantic::kInt32 ||
            semantic() == MachineSemantic::kInt64;
   }
   bool IsUnsigned() {
     return semantic() == MachineSemantic::kUint32 ||
            semantic() == MachineSemantic::kUint64;
   }
+  bool IsWidestSize() {
+    switch (representation()) {
+      case MachineRepresentation::kWord64:
+      case MachineRepresentation::kFloat64:
+      case MachineRepresentation::kTagged:
+        return true;
+      case MachineRepresentation::kNone:
+      case MachineRepresentation::kBit:
+      case MachineRepresentation::kWord8:
+      case MachineRepresentation::kWord16:
+      case MachineRepresentation::kWord32:
+      case MachineRepresentation::kFloat32:
+        return false;
+      default:
+        break;
+    }
+    // kSimd128 is not in use yet.
+    UNREACHABLE();
+    return false;
+  }
 
   static MachineRepresentation PointerRepresentation() {
     return (kPointerSize == 4) ? MachineRepresentation::kWord32
                                : MachineRepresentation::kWord64;
   }
   static MachineType Pointer() {
     return MachineType(PointerRepresentation(), MachineSemantic::kNone);
   }
   static MachineType IntPtr() {
     return (kPointerSize == 4) ? Int32() : Int64();
@@ skipping 126 matching lines @@
   UNREACHABLE();
   return -1;
 }
 
 typedef Signature<MachineType> MachineSignature;
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MACHINE_TYPE_H_