Add HLoadRoot hydrogen instruction.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 8d08df4a33258584ce8b87c92e61be82dd4d247d..8e3dc55e7f48416f80d0a6ce70aec013614ee9cb 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -2175,6 +2175,11 @@ LInstruction* LChunkBuilder::DoLoadFunctionPrototype(
 }
 
 
+LInstruction* LChunkBuilder::DoLoadRoot(HLoadRoot* instr) {
+  return DefineAsRegister(new(zone()) LLoadRoot);
+}
+
+
 LInstruction* LChunkBuilder::DoLoadExternalArrayPointer(
     HLoadExternalArrayPointer* instr) {
   LOperand* input = UseRegisterAtStart(instr->value());