[mips] Marked up instructions added in MIPS32 and tested that IAS for -mcpu=mips2 does not accept them

Summary:
To limit the number of tests required, only one 32-bit and one 64-bit ISA
prior to MIPS32/MIPS64 are explicitly tested.

Depends on D3695

Reviewers: vmedic

Differential Revision: http://reviews.llvm.org/D3696