[Pipelines] Add LoopSink and DivRemPairs to LTO post-link pipeline

As pointed out in D148010, these passes are missing from the LTO
post-link pipeline. They are present in the pre-link pipeline,
but LoopSink is completely useless there (it will always be fully
undone by LICM post-link) and DivRemPairs is mostly useless
(I believe most of what it does will be undone by InstCombine).

I've not added RelLookupTableConverterPass, because it's also
disabled in the LTO pre-link pipeline, with a comment that there
is an unresolved issue with full LTO.

Compile-time impact of the extra passes is minimal. Of course,
LoopSink will have a larger impact in PGO builds.

Differential Revision: https://reviews.llvm.org/D148343