[mips][mips64r6] daddi is not available on MIPS64r6

Summary:
It's not emitted by the code generator so we only need assembler tests.

Also added missing daddi aliases from dsub mnemonics, and removed a couple
duplicate dsub tests.

Depends on D4112

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4113