Don't do uint64_t(1) << 64 in maxUIntN.

Summary:
This shift is undefined behavior (and, as compiled by clang, gives the
wrong answer for maxUIntN(64)).

Reviewers: mkuper

Subscribers: llvm-commits, jroelofs, rsmith

Differential Revision: https://reviews.llvm.org/D22430

llvm-svn: 275656