MIPS: Add HLoadRoot hydrogen instruction.

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index a7319aeb5e70b3477d1903f4f905a0f612d37044..4f18a5e9d11d40702d53fcf52caf67efbc4ad10f 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -2071,6 +2071,11 @@ LInstruction* LChunkBuilder::DoLoadFunctionPrototype(
 }
 
 
+LInstruction* LChunkBuilder::DoLoadRoot(HLoadRoot* instr) {
+  return DefineAsRegister(new(zone()) LLoadRoot);
+}
+
+
 LInstruction* LChunkBuilder::DoLoadExternalArrayPointer(
     HLoadExternalArrayPointer* instr) {
   LOperand* input = UseRegisterAtStart(instr->value());