[NFCI] SimplifyCFGPass: mergeEmptyReturnBlocks(): use DeleteDeadBlocks()

In this case, it does the same thing as the original pattern does.

SimplifyCFG has a few lurking miscompilations about deleting blocks that
have their address taken, and consistently using DeleteDeadBlocks() instead
 of a hand-rolled pattern will allow to weed those cases out easierly.