refactor AssignmentFunctors.h, unify with existing scalar_op

Reference issue

What does this implement/fix?

This cleans up the assignment functors, which contain a lot of redundant code (encountered on the other MR I'm working on). This has the side effect of making the behavior of compound assignments, e.g. a += b, consistent with the simple assignment a = a + b. One prominent example is that for vectorized integer division, we check for division by zero in scalar_quotient_op, but not div_assign_op.

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