[LazyCallGraph] Ignore empty RefSCCs rather than shift RefSCCs when removing dead functions

This is in preparation for D115545 which attempts to delete discardable functions if they are unused. With that change, shifting RefSCCs becomes noticeable in compile time. This change makes the LCG update negligible again.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D116776