Add HLoadRoot hydrogen instruction.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 3a7366e324699dfdfb8ae6b3f42760298b0984e0..4efb3032e5b84fbb6a964cf0a3e9ae57b198f4f5 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2075,6 +2075,11 @@ LInstruction* LChunkBuilder::DoLoadFunctionPrototype(
 }
 
 
+LInstruction* LChunkBuilder::DoLoadRoot(HLoadRoot* instr) {
+  return DefineAsRegister(new(zone()) LLoadRoot);
+}
+
+
 LInstruction* LChunkBuilder::DoLoadExternalArrayPointer(
     HLoadExternalArrayPointer* instr) {
   LOperand* input = UseRegisterAtStart(instr->value());