[mips] Improve robustness of some tests.

Summary:
This is done by removing some hardcoded registers like $at or expecting a single digit register to be selected.

Contains work done by Matheus Almeida.

Reviewers: matheusalmeida, dsanders

Reviewed By: dsanders

Subscribers: tomatabacu

Differential Revision: http://reviews.llvm.org/D4227

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215640 91177308-0d34-0410-b5e6-96231b3b80d8