Bugfix for splitting critical edges in shrink wrapping

Summary:
Fix issue with splitting critical edges originating at
the same BB in ShrinkWrapping::splitFrontierCritEdges.

Splitting of critical edges originating at the same FromBB
wasn't handled correctly as the Frontier at index corresponding
to FromBB was overwritten with basic blocks created for
multiple DestinationBBs.

(cherry picked from FBD23232398)