[OpenMPOpt][HideMemTransfersLatency] Get values stored in offload arrays

getValuesInOffloadArrays goes through the offload arrays in __tgt_target_data_begin_mapper getting the values stored in them before the call is issued.

call void @__tgt_target_data_begin_mapper(arg0, arg1,
    i8** %offload_baseptrs, i8** %offload_ptrs, i64* %offload_sizes,
...)

Diferential Revision: https://reviews.llvm.org/D86300