[mips] Update tests with encoding information for slt, slti, sltiu and sltu.

Summary:
Also renamed non-portable register names (e.g. $t2) so that we don't end up
with a different encoding for what appears to be an equivalent instruction.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3505

llvm-svn: 207655