[LoopRotate] Don't split loop pass manager

After D99249 we use three different loop pass managers for LICM,
LoopRotate and LICM+LoopUnswitch. This happens because LazyBFI
and LazyBPI are not preserved by LoopRotate (note that D74640
is no longer needed). Avoid this by marking them as preserved.

My understanding of D86156 is that it is okay to simply preserve
them (which LoopUnswitch already does for the same reason) and
rely on callbacks to deal with deleted blocks.

Differential Revision: https://reviews.llvm.org/D99843