Use the return of readBytes to find out if we are at the end of the stream.

Use the return of readBytes to find out if we are at the end of the stream.

This allows the removal of isObjectEnd and opens the way for reading 64 bits
at a time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221804 91177308-0d34-0410-b5e6-96231b3b80d8