[mips] Fix ehframe-indirect.ll test.

Summary:
-check-prefix replaces the default CHECK prefix rather than adding to it and
must be explicitly re-added.

Also added the N32 cases.

Reviewers: petarj

Reviewed By: petarj

Subscribers: tberghammer, llvm-commits

Differential Revision: http://reviews.llvm.org/D9668

llvm-svn: 237790