[PM] Teach the loop PM to run LoopSimplify prior to the loop pipeline.

This adds the last remaining core feature of the loop pass pipeline in
the new PM and removes the last of the really egregious hacks in the
LICM tests.

Sadly, this requires really substantial changes in the unittests in
order to provide and maintain simplified loops. This is particularly
hard because for example LoopSimplify will try to fold undef branches to
an ideal direction and simplify the loop accordingly.

Differential Revision: https://reviews.llvm.org/D28766

llvm-svn: 292709