[OpenMPOpt][HideMemTransfersLatency] Moving the 'wait' counterpart of __tgt_target_da...

[OpenMPOpt][HideMemTransfersLatency] Moving the 'wait' counterpart of __tgt_target_data_begin_mapper

canBeMovedDownwards checks if the "wait" counterpart of __tgt_target_data_begin_mapper can be moved downwards, returning a pointer to the instruction that might require/modify the data transferred, and returning null it the movement is not possible or not worth it. The function splitTargetDataBeginRTC receives that returned instruction and instead of moving the "wait" it creates it at that point.

Differential Revision: https://reviews.llvm.org/D86155