Adds macros to the MIPS assembler for detecting overflow.

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
===================================================================
--- runtime/vm/assembler_mips.h	(revision 20999)
+++ runtime/vm/assembler_mips.h	(working copy)
@@ -506,6 +506,16 @@
 
   // Macros in alphabetical order.
 
+  // Addition of rs and rt with the result placed in rd.
+  // After, ro < 0 if there was signed overflow, ro >= 0 otherwise.
+  // Clobbers TMP.
+  // Clobbers T9 if rd, rs, rt are the same register.
+  void AdduDetectOverflow(Register rd, Register rs, Register rt, Register ro);
+
+  // Loads the immediate value into T8, and calls AdduDetectOverflow.
+  void AddiuDetectOverflow(Register rt, Register rs, const Immediate& imm,
+                           Register ro);
+
   void Branch(const ExternalLabel* label) {
     LoadImmediate(TMP, label->address());
     jr(TMP);