[Reassociate] Remove FIXME from looptest.ll (NFC)

Summary: The loop invariant add (i+j) is reassoicated, I think the FIXME can be removed, because this is what the test case tries to check (AFAIK). I also changed the test to use FileCheck.

Reviewers: mcrosier, davide

Reviewed By: mcrosier, davide

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D39424

llvm-svn: 317000