Subzero: Clean up some uses of *_cast<>.

src/IceTargetLoweringX8664Traits.h

Index: src/IceTargetLoweringX8664Traits.h
diff --git a/src/IceTargetLoweringX8664Traits.h b/src/IceTargetLoweringX8664Traits.h
index 50a1544920ea3fbebabf9b762364e8e05f45c8d3..336e3dfcacb1f31ecf9fed626c19d32de99e4d20 100644
--- a/src/IceTargetLoweringX8664Traits.h
+++ b/src/IceTargetLoweringX8664Traits.h
@@ -784,9 +784,7 @@ public:
   /// representation of the vector.
   static Type getInVectorElementType(Type Ty) {
     assert(isVectorType(Ty));
-    size_t Index = static_cast<size_t>(Ty);
-    (void)Index;
-    assert(Index < TableTypeX8664AttributesSize);
+    assert(Ty < TableTypeX8664AttributesSize);
     return TableTypeX8664Attributes[Ty].InVectorElementType;
   }
 
@@ -841,9 +839,8 @@ public:
   /// @}
 
   static Cond::BrCond getIcmp32Mapping(InstIcmp::ICond Cond) {
-    size_t Index = static_cast<size_t>(Cond);
-    assert(Index < TableIcmp32Size);
-    return TableIcmp32[Index].Mapping;
+    assert(Cond < TableIcmp32Size);
+    return TableIcmp32[Cond].Mapping;
   }
 
   static const struct TableTypeX8664AttributesType {
@@ -1004,7 +1001,7 @@ public:
     static SpillVariable *create(Cfg *Func, Type Ty, SizeT Index) {
       return new (Func->allocate<SpillVariable>()) SpillVariable(Ty, Index);
     }
-    const static OperandKind SpillVariableKind =
+    constexpr static auto SpillVariableKind =
         static_cast<OperandKind>(kVariable_Target);
     static bool classof(const Operand *Operand) {
       return Operand->getKind() == SpillVariableKind;