rework the Bitstream reader to actually work a machine word at a time, instead of 32-bits at a time.
This cuts in half the number of virtual methods called to refill that word when compiling on a 64-bit
host, and will make 64-bit read operations faster.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173072 91177308-0d34-0410-b5e6-96231b3b80d8