LegalizeDAG: Implement PROMOTE for ISD::BITREVERSE

Summary:
This operation is promoted the same way was ISD::BSWAP.  This will
prevent a regression in test/Target/AMDGOU/bitreverse.ll when i16
support is implemented.

Reviewers: bogner, hfinkel

Subscribers: hfinkel, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D25202

llvm-svn: 284163