Have DataLayout::isLegalInteger() accept uint64_t

While not strictly necessary, since we don't support large integer
types, this avoids bugs due to silent truncation from uint64_t to a
32-bit unsigned (e.g. DL.isLegalInteger(DL.getTypeSizeInBits(Ty) )

This fixes PR26972.

Differential Revision: http://reviews.llvm.org/D18258

llvm-svn: 263850