Add a kSimd128 machine type for Turbofan.

src/machine-type.h

Index: src/machine-type.h
diff --git a/src/machine-type.h b/src/machine-type.h
index 97f6ae3bbdb92342aaeed48d8d63badf4f323087..1085657894a94ea5a8d351496fd721d7861094a6 100644
--- a/src/machine-type.h
+++ b/src/machine-type.h
@@ -24,6 +24,7 @@ enum class MachineRepresentation : uint8_t {
   kWord64,
   kFloat32,
   kFloat64,
+  kSimd128,
   kTagged
 };
 
@@ -84,6 +85,9 @@ class MachineType {
     return MachineType(MachineRepresentation::kFloat64,
                        MachineSemantic::kNumber);
   }
+  static MachineType Simd128() {
+    return MachineType(MachineRepresentation::kSimd128, MachineSemantic::kNone);
+  }
   static MachineType Int8() {
     return MachineType(MachineRepresentation::kWord8, MachineSemantic::kInt32);
   }
@@ -143,6 +147,9 @@ class MachineType {
   static MachineType RepFloat64() {
     return MachineType(MachineRepresentation::kFloat64, MachineSemantic::kNone);
   }
+  static MachineType RepSimd128() {
+    return MachineType(MachineRepresentation::kSimd128, MachineSemantic::kNone);
+  }
   static MachineType RepTagged() {
     return MachineType(MachineRepresentation::kTagged, MachineSemantic::kNone);
   }
@@ -187,6 +194,8 @@ inline int ElementSizeLog2Of(MachineRepresentation rep) {
     case MachineRepresentation::kWord64:
     case MachineRepresentation::kFloat64:
       return 3;
+    case MachineRepresentation::kSimd128:
+      return 4;
     case MachineRepresentation::kTagged:
       return kPointerSizeLog2;
     default: