[MBP] Enable duplicating return block to remove jump to return

Sometimes LLVM generates branch to return instruction, like PR63227.

It is because in function MachineBlockPlacement::canTailDuplicateUnplacedPreds
we avoid duplicating a BB into another already placed BB to prevent destroying
computed layout. But if the successor BB is a return block, duplicating it will
only reduce taken branches without hurt to any other branches.

Differential Revision: https://reviews.llvm.org/D153093