Properly check for self-assignments

[karlrupp]

Special cases like x = prod(A,x) require additional attention. There are checks in ViennaCL for this case, but they don't provide a unified behavior: Some create a temporary vector (good), others get stuck in asserts() (not so good). Some checks are overly restrictive when ranges are used.

[karlrupp]

Checked the most common operations, a lot of the problems mentioned in the issue have been resolved with the introduction of the scheduler and op_executor. There are still a couple of corner cases left, but these are rather uncommon in practice. This is likely to be resolved with full scheduler integration, hence postponing to 1.6.0.

[karlrupp]

Currently no known issues, but a separate regression test desirable. Removing 'bug' label and scheduling for 1.6.1.