Revert earlier change removing setPreservesCFG from instcombine (r221223) and
change LoopSimplifyPass to be !isCFGOnly.  The motivation for the earlier patch
(r221223) was that LoopSimplify is not preserved by instcombine though
setPreservesCFG indicates that it is.  This change fixes the issue
by making setPreservesCFG no longer imply LoopSimplifyPass, and is therefore less
invasive.