Adds branch instructions and labels to the MIPS simulator, assembler, disassembler.

runtime/vm/constants_mips.h

Index: runtime/vm/constants_mips.h
===================================================================
--- runtime/vm/constants_mips.h	(revision 20399)
+++ runtime/vm/constants_mips.h	(working copy)
@@ -174,6 +174,8 @@
   kInstrBits = 26,
   kBreakCodeShift = 5,
   kBreakCodeBits = 20,
+
+  kBranchOffsetMask = 0x0000ffff,
 };
 
 
@@ -366,6 +368,10 @@
     return static_cast<SpecialFunction>(Bits(kFunctionShift, kFunctionBits));
   }
 
+  inline RtRegImm RegImmFnField() const {
+    return static_cast<RtRegImm>(Bits(kRtShift, kRtBits));
+  }
+
   inline bool IsBreakPoint() {
     return (OpcodeField() == SPECIAL) && (FunctionField() == BREAK);
   }