[mips] Correct tests that are meant to test valid assembly. They were actually rejected by GAS.

Summary:
I've noticed a bug in my test generator script that caused 64-bit objects
to be disassembled as if it were using the O32 ABI, giving the wrong register
names. As a result, it generated assembly files that are rejected by GAS when
assembling for the correct ABI. This was caused by the generator setting the
ELF e_flags incorrectly before disassembling the object.

This patch corrects the invalid tests that have already been committed by
replacing the ABI-dependent register names with numeric registers. In addition
to fixing the tests this allows the 32-bit and 64-bit ISA tests to be easily diffed
to produce the invalid-*.s tests which test that instructions defined in later ISA's
are not accepted.

Depends on D3648

Reviewers: vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3649