[MemCpyOpt] Fix metadata merging during call slot optimization

Call slot optimization currently merges the metadata between the
call and the load. However, we also need to merge in the metadata
of the store.

Part of the reason why we might have gotten away with this
previously is that usually the load and the store are the same
instruction (a memcpy), this can only happen if call slot
optimization occurs on an actual load/store pair.

This addresses the issue reported in
https://reviews.llvm.org/D115615#3251386.

Differential Revision: https://reviews.llvm.org/D117679