MIPS: Improve code aging sequence.

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 58a8a61f0d81d9ba41247c0508fac906b2d9657a..2ee70ff45e3e42dbbdaf77f87e0ef6e8ae573003 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -159,9 +159,9 @@ bool LCodeGen::GeneratePrologue() {
       // The following three instructions must remain together and unmodified
       // for code aging to work properly.
       __ Push(ra, fp, cp, a1);
-      // Add unused load of ip to ensure prologue sequence is identical for
+      // Add unused nop to ensure prologue sequence is identical for
       // full-codegen and lithium-codegen.
-      __ LoadRoot(at, Heap::kUndefinedValueRootIndex);
+      __ nop(Assembler::CODE_AGE_SEQUENCE_NOP);
       // Adj. FP to point to saved FP.
       __ Addu(fp, sp, Operand(2 * kPointerSize));
     }