[Passes] Use MemorySSA for LICM during LTO.

Split off from D102345 to commit this separately from other changes in
the patch. This aligns the behavior of the new PM with the legacy PM
for LTO, with respect to running LICM.

Together with the remaining changes in D102345, this fixes new PM
regressions where we fail to vectorize loops that are vectorized with
the legacy PM.