[mips][microMIPS] Implement 16-bit instructions registers including ZERO instead of S0

Implement microMIPS 16-bit instructions register set: $0, $2-$7 and $17.

Differential Revision: http://reviews.llvm.org/D5780


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222652 91177308-0d34-0410-b5e6-96231b3b80d8