Port PlaceSafepoints pass to the new pass manager

Port PlaceSafepoints pass to the new pass manager

This patch ports the PlaceSafepoints pass to the new pass manager as it is used by .NET/Mono. Compatibility with the legacy pass manager is maintained by adding PlaceSafepointsLegacyPass. This pass also depends on PlaceBackedgeSafepointsLegacyPass, which has been kept in the legacy-only variant, since it is apparently used only from PlaceSafepointsPass. It has been renamed, though, to indicate its legacy interface.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D136163