Add UBSAN build option and fix undefined behaviour errors.

src/IceTargetLoweringX8664Traits.h

Index: src/IceTargetLoweringX8664Traits.h
diff --git a/src/IceTargetLoweringX8664Traits.h b/src/IceTargetLoweringX8664Traits.h
index a84dbc486e35fa79d8a8276a0ba30635ec99f856..6ae9ac949d19255091ed7e3ea02ae67b159f96a1 100644
--- a/src/IceTargetLoweringX8664Traits.h
+++ b/src/IceTargetLoweringX8664Traits.h
@@ -565,7 +565,7 @@ template <> struct MachineTraits<TargetX8664> {
     X86Operand &operator=(const X86Operand &) = delete;
 
   public:
-    enum OperandKindX8664 { k__Start = ::Ice::Operand::kTarget, kMem, kSplit };
+    enum OperandKindX8664 { k__Start = ::Ice::Operand::kTarget0, kMem, kSplit };
     using ::Ice::Operand::dump;
 
     void dump(const Cfg *, Ostream &Str) const override;
@@ -685,7 +685,7 @@ template <> struct MachineTraits<TargetX8664> {
       return new (Func->allocate<SpillVariable>()) SpillVariable(Ty, Index);
     }
     const static OperandKind SpillVariableKind =
-        static_cast<OperandKind>(kVariable_Target);
+        static_cast<OperandKind>(kVariable_Target0);
     static bool classof(const Operand *Operand) {
       return Operand->getKind() == SpillVariableKind;
     }