Fix some errors introduced by rL370563 which were not exposed on my local machine.

Fix some errors introduced by rL370563 which were not exposed on my local machine.
1. zlib::compress accept &size_t but the param is an uint64_t.
2. Some systems don't have zlib installed. Don't use compression by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370564 91177308-0d34-0410-b5e6-96231b3b80d8