recommit r173072 (preparing bitstream reader to read a machine word at a time,

recommit r173072 (preparing bitstream reader to read a machine word at a time,
instead of always 32-bits at a time) with two changes:

1. Make Read(0) always return zero without affecting the state of our cursor.
2. Hack word_t to always be 32 bits, as staging.

These two caveats will change shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174800 91177308-0d34-0410-b5e6-96231b3b80d8