[mips] Test that IAS for -mcpu=mips5 does not accept MIPS64 insns and -mcpu=mips(5|64) does not accept MIPS64r2

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

rdhwr has been deliberately left without an ISA annotation for now. This is
because the assembler and CodeGen disagree on when the instruction is
available. Strictly speaking, it is only available in MIPS32r2 and
MIPS64r2. However, it is emulated by a kernel trap on earlier ISA's and is
necessary for TLS so CodeGen should emit it on older ISA's too.

Depends on D3697

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3698