[DSE] Simplify based on opaque pointers

Given the transition to opaque pointers we no longer need to emit
some pointer casts. Int8PtrTy was set up to be a ptr in same address
space a OrigDest, making the first CreatePointerCast dead. And then
NewDestGEP will end up having the same type as OrigDest, making the
second CreatePointerCast dead.