[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209418 91177308-0d34-0410-b5e6-96231b3b80d8