[mips] Clean up the CodeGen/Mips/inlineasmmemop.ll test. NFC.

Summary:
Improve comments and remove a redundant attribute list.
There are no functional changes (to the CHECK's or to the code).

Part of these changes were suggested in http://reviews.llvm.org/D6637.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6705