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

src/machine-type.h

Index: src/machine-type.h
diff --git a/src/machine-type.h b/src/machine-type.h
index 08786100b05c2ceaf88001c48ce871fc26499da8..3bea9568ee0325cf214b0be21fb23d8e2fb61426 100644
--- a/src/machine-type.h
+++ b/src/machine-type.h
@@ -66,6 +66,26 @@ class MachineType {
     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