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 +506,12 @@
 
   // 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.

[regis, 2013/04/06 00:33:24]

By definition, TMP can be clobbered by any asm macro.
You should add a more detailed comment about register usage, otherwise the
caller will waste a register.
You should explain when a scratch register is required and when not.

[zra, 2013/04/08 16:45:25]

Done.

+  void AdduDetectOverflow(Register rd, Register rs, Register rt, Register ro,
+                          Register scratch);
+
   void Branch(const ExternalLabel* label) {
     LoadImmediate(TMP, label->address());
     jr(TMP);