[mips][mips64r6] Add bitswap, and dbitswap

Summary: Depends on D3728

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3729

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208877 91177308-0d34-0410-b5e6-96231b3b80d8