[MemCpyOpt] Handle unusual memcpy element type

Apparently, it is legal to use memcpy/memset with pointer types
other than i8*. Prior to 81fcdae6
this case was silently miscompiled, as the i8 offset calculation
was performed on some other type. Now it would crash due to a
type mismatch. Fix this by inserting an explicit bitcast to i8*.