[mips][mips64r6] Test that paired single instructions are invalid

Summary:
These emit the 'unknown instruction' instead of the correct error
because they have not been implemented in LLVM for any MIPS ISA.

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3841

llvm-svn: 209418