Remove Array.offset

[inducer]

trafficstars

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