Dex verifier: Add offset-to-pointer helper.

The field `begin_` is somewhat ambiguous in presence of dex containers.

Use more semantically descriptive OffsetToPtr helper to avoid it.

As a bonus, this adds range DCHECK to all such accesses.

Test: ./art/test.py -b --host --optimizing --64
Change-Id: Ia29e6901512e1a12b30c6c6bcc5c6a1ea09576a6