SERVER-72233 simplified the explain output for Evaluation nodes to concisely list projections at the top level. We can simplify that output further in cases where the only node in the Evaluation path is a simple constant or variable.

For example, what is currently printed as

Evaluation [{val_prev}]
|     Variable [valIn]

could be simplified to

Evaluation[{val_prev = valIn}]

and what is current printed as 

Evaluation [{val_prev}]
|     Const [1]

could be simplified to

Evaluation[{val_prev = 1}]