BitstreamReader hasn't aged well.  It's been hacked on by various people and
has past the point of making sense.  Lets tidy things up: first step, moving
a ton of big functions out of line.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172904 91177308-0d34-0410-b5e6-96231b3b80d8