Remove Array.offset

`offset` has no business being in the IR. It's a concern that's specific entrypoints in OpenCL and *nothing* else. So it should be a concern for *just that* target.

How does this sound:

* [ ] As a first step, change the default for `offset` to `lp.auto`, deprecate passing anything else. This is a change in behavior, but it should be safe. For `offset == 0`, steer people towards adding a `lp.NoOffsetInCL` tag, which avoids the cost of the additional offset argument.
* [ ] Deprecate accessing `ary.offset`.
* [ ] After the deprecation expires, remove `ary.offset`