I was studying changes to BackendUtil introduced by D71913 and found out that
- Removing invocation of LTT pass in BackendUtil.cpp doesn't break any test case including thinlto-distributed-type-metadata.cpp
- When combined index doesn't have skipModuleByDistributedBackendflag we run normal thin LTO pipeline which removes type.test/assume sequences either in WPD or in LTT.
- When combined index does have skipModuleByDistributedBackend flag we end up with empty module without invoking backend.
So adding LTT pass in BackendUtil seems to be not needed.