[LPM] Apply a really big hammer to fix PR18688 by recursively reforming
LCSSA when we promote to SSA registers inside of LICM.

Currently, this is actually necessary. The promotion logic in LICM uses
SSAUpdater which doesn't understand how to place LCSSA PHI nodes.
Teaching it to do so would be a very significant undertaking. It may be
worthwhile and I've left a FIXME about this in the code as well as
starting a thread on llvmdev to try to figure out the right long-term
solution.

For now, the PR needs to be fixed. Short of using the promition
SSAUpdater to place both the LCSSA PHI nodes and the promoted PHI nodes,
I don't see a cleaner or cheaper way of achieving this. Fortunately,
LCSSA is relatively lazy and sparse -- it should only update
instructions which need it. We can also skip the recursive variant when
we don't promote to SSA values.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200612 91177308-0d34-0410-b5e6-96231b3b80d8