[clang][ExprConst][NFC] Replace typecheck+castAs with getAs

Author: tbaederr

clang/lib/AST/ExprConstant.cpp

@@ -14014,8 +14014,8 @@ bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
     QualType Ty = E->getTypeOfArgument();
     // If the vector has a fixed size, we can determine the number of elements
     // at compile time.
-    if (Ty->isVectorType())
-      return Success(Ty->castAs<VectorType>()->getNumElements(), E);
+    if (const auto *VT = Ty->getAs<VectorType>())
+      return Success(VT->getNumElements(), E);
 
     assert(Ty->isSizelessVectorType());
     if (Info.InConstantContext)