Read 64 bits at a time in the bitcode reader.

The reading of 64 bit values could still be optimized, but at least this cuts
down on the number of virtual calls to fetch more data.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221865 91177308-0d34-0410-b5e6-96231b3b80d8