Avoid UB in maxIntN(64).

Summary:
Previously we were relying on 2's complement underflow in an int64_t.
Now we cast to a uint64_t so we explicitly get the behavior we want.

Reviewers: rnk

Subscribers: dylanmckay, llvm-commits

Differential Revision: https://reviews.llvm.org/D22445

llvm-svn: 275722