Allow loopback between steps and other job elements

loopback between steps and other job elements is prohibited by the batch spec (see quote below), but sometimes applications can benefit from this construct, since loop is a common design choice and construct. Need to evaluate how to allow it in those cases.

8.9.3 Loop definition

The specification prohibits ‘next’ and ‘to’ attribute values that result in a “loop”. More precisely, this means that no execution element can be transitioned to twice within a single job execution.
This wording is purposely written this way rather than merely saying no execution element can be executed twice within a single job execution. Say “step1” executed to completion during an initial execution which ultimately failed, and upon restart we transitioned past “step1” without executing it since it had already completed, but we subsequently transitioned (back) to “step1”. This may only be a single execution of “step1” during a single job execution, but it still violates the looping prohibition.

The runtime may detect potential loops in an initial validation phase, as described in section 13.1, or may only detect loops once they occur.