Fixes #3690

Summary/Motivation:

#3690 reported a bug in the simulator where if you had a differential equation of the form A*dxdt + B == RHS it would be handled differently than dxdt*A + B == RHS. Pyomo.DAE checks for certain forms that a differential equation may have been written in and tries to permute those forms to a standard separable format expected by the integrators. At one point I think product expressions would have been rearranged such that if A was not potentially variable it would have ALWAYS been moved to arg[0] in the ProductExpression so the Simulator only ever checked for A*dxdt. When we stopped doing automatic expression mangling in core Pyomo the assumption that the simulator didn't have to check for dxdt*A was no longer valid and the bug was introduced. This PR fixes it by ensuring the Simulator is checking for both cases.

Changes proposed in this PR:

• Fix bug in simulator code for identifying separable differential equations.

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