Add a HBreak instruction for debugging

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index a2a6151d1947999c99a5c3074ad4b4cd36a5b45c..9a1ce9800993d683dc04ea27bee12b54b70c9168 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -1863,6 +1863,11 @@ void LCodeGen::EmitBranch(int left_block, int right_block, Condition cc) {
 }
 
 
+void LCodeGen::DoDebugBreak(LDebugBreak* instr) {
+  __ int3();
+}
+
+
 void LCodeGen::DoBranch(LBranch* instr) {
   int true_block = chunk_->LookupDestination(instr->true_block_id());
   int false_block = chunk_->LookupDestination(instr->false_block_id());