MIPS: Add HLoadRoot hydrogen instruction.

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index 80c20512a050e9a78be660d83df6785bdcbce494..6f82e1bd75d77d9362318cec51cb23be206f7cd1 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -3002,6 +3002,12 @@ void LCodeGen::DoLoadFunctionPrototype(LLoadFunctionPrototype* instr) {
 }
 
 
+void LCodeGen::DoLoadRoot(LLoadRoot* instr) {
+  Register result = ToRegister(instr->result());
+  __ LoadRoot(result, instr->index());
+}
+
+
 void LCodeGen::DoLoadExternalArrayPointer(
     LLoadExternalArrayPointer* instr) {
   Register to_reg = ToRegister(instr->result());