Implement asPartialDefinition for IR dataflow nodes

[MathiasVP]

The current asPartialDefinition for dataflow nodes for the IR doesn't always produce the output one would expect when looking at the AST versions. In particular:

1. It doesn't always produce a result (even in cases where we expect a result to be produced)
2. When it produces a result, it returns the expression that defines the partial definition (i.e., the right hand side of a partial definition)

The predicate isn't used very much in our codebase, but it's still nice to be able to track how well the IR is behaving like the AST. This PR:

1. adds a couple of tests that check whether the partial definitions are present, and the difference between the IR and AST
2. Fixes the easiest cases so that the IR produces the correct expression corresponding to the partial definition

[rdmarsh2]

LGTM