[mips][mips64r6] prefx is not available on MIPS32r6/MIPS64r6

Summary: We haven't implemented this instruction so we only add a test case.

Reviewers: vmedic, zoran.jovanovic, jkolek

Reviewed By: jkolek

Differential Revision: http://reviews.llvm.org/D4004

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210779 91177308-0d34-0410-b5e6-96231b3b80d8