Add a HBreak instruction for debugging

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index d6c3d9580e72842c9384c5f09103c4efaadd275c..747d8e73d48c640766c4f9e6830894c62f8d94b8 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -89,6 +89,7 @@ class LCodeGen;
   V(ConstantI)                                  \
   V(ConstantT)                                  \
   V(Context)                                    \
+  V(DebugBreak)                                 \
   V(DeclareGlobals)                             \
   V(DeleteProperty)                             \
   V(Deoptimize)                                 \
@@ -1172,6 +1173,12 @@ class LBranch: public LControlInstruction<1, 0> {
 };
 
 
+class LDebugBreak: public LTemplateInstruction<0, 0, 0> {
+ public:
+  DECLARE_CONCRETE_INSTRUCTION(DebugBreak, "break")
+};
+
+
 class LCmpMapAndBranch: public LTemplateInstruction<0, 1, 0> {
  public:
   explicit LCmpMapAndBranch(LOperand* value) {