[Bitcode] AtEndOfStream should only check against the size if it's known.

This avoids an issue where AtEndOfStream mistakenly returns true at the /start/ of
a stream.

(In the rare case that the size is known and actually 0, the slow path will still
handle it correctly.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221840 91177308-0d34-0410-b5e6-96231b3b80d8