Add UBSAN build option and fix undefined behaviour errors.

src/IceInstX86Base.h

 //===- subzero/src/IceInstX86Base.h - Generic x86 instructions -*- C++ -*--===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 19 matching lines @@
 
 template <class Machine> class InstX86Base : public InstTarget {
   InstX86Base<Machine>() = delete;
   InstX86Base<Machine>(const InstX86Base &) = delete;
   InstX86Base &operator=(const InstX86Base &) = delete;
 
 public:
   using Traits = MachineTraits<Machine>;
 
   enum InstKindX86 {
-    k__Start = Inst::Target,
+    k__Start = Inst::Target0,
     Adc,
     AdcRMW,
     Add,
     AddRMW,
     Addps,
     Addss,
     Adjuststack,
     And,
     AndRMW,
     Blendvps,
@@ skipping 3157 matching lines @@
           &InstX86Base<Machine>::Traits::Assembler::psrl};                     \
   }                                                                            \
   }
 
 } // end of namespace X86Internal
 } // end of namespace Ice
 
 #include "IceInstX86BaseImpl.h"
 
 #endif // SUBZERO_SRC_ICEINSTX86BASE_H