[mips] [IAS] Reformat mips-expansions.s. NFC.

Summary:
Make mips-expansions.s more readable by grouping the instructions with their respective CHECK's.
This test is going to get a lot bigger soon and it will become essentially unreadable if the current formatting is kept.

I've also made the comments more useful and accurate, and I've restricted the RUN lines to under 80 columns.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10089

llvm-svn: 238817