[CFLGraph][NFC] Simplify/reorder switch in visitConstantExpr

Reviewers: hfinkel, efriedma, spatel, dsanders, Danil, rjmccall

Reviewed By: rjmccall

Subscribers: dberlin, llvm-commits

Differential Revision: https://reviews.llvm.org/D46259

llvm-svn: 331312