[mips] Fix aui/daui/dahi/dati for MIPSR6

For compatiblity with binutils, define these instructions to take
two registers with a 16bit unsigned immediate. Both of the registers
have to be same for dahi and dati.

Reviewers: dsanders, zoran.jovanovic

Differential Review: https://reviews.llvm.org/D21473

llvm-svn: 284218