[Analysis] Support bitreverse in -demanded-bits pass

Summary:
* Add a bitreverse case in the demanded bits analysis pass.
* Add tests for the bitreverse (and bswap) intrinsic in the
  demanded bits pass.
* Add a test case to the BDCE tests: that manipulations to
  high-order bits are eliminated once the bits are reversed
  and then right-shifted.

Reviewers: mkuper, jmolloy, hfinkel, trentxintong

Reviewed By: jmolloy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31857

llvm-svn: 300215