[mips] Range check simm5.

Summary:
We can't check the error message for this one because there's another lw/sw
available that covers a larger range. We therefore check the transition
between the two sizes.

Reviewers: vkalintiris

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D18144

llvm-svn: 264054