Add UBSAN build option and fix undefined behaviour errors.

src/IceInstMIPS32.h

 //===- subzero/src/IceInstMIPS32.h - MIPS32 machine instrs --*- C++ -*-===    //
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 15 matching lines @@
 
 class TargetMIPS32;
 
 /// Base class for Mips instructions.
 class InstMIPS32 : public InstTarget {
   InstMIPS32() = delete;
   InstMIPS32(const InstMIPS32 &) = delete;
   InstMIPS32 &operator=(const InstMIPS32 &) = delete;
 
 public:
-  enum InstKindMIPS32 { k__Start = Inst::Target, Ret };
+  enum InstKindMIPS32 { k__Start = Inst::Target0, Ret };
 
   static const char *getWidthString(Type Ty);
 
   void dump(const Cfg *Func) const override;
 
 protected:
   InstMIPS32(Cfg *Func, InstKindMIPS32 Kind, SizeT Maxsrcs, Variable *Dest)
       : InstTarget(Func, static_cast<InstKind>(Kind), Maxsrcs, Dest) {}
   static bool isClassof(const Inst *Inst, InstKindMIPS32 MyKind) {
     return Inst->getKind() == static_cast<InstKind>(MyKind);
@@ skipping 24 matching lines @@
   void dump(const Cfg *Func) const override;
   static bool classof(const Inst *Inst) { return isClassof(Inst, Ret); }
 
 private:
   InstMIPS32Ret(Cfg *Func, Variable *RA, Variable *Source);
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEINSTMIPS32_H