When a $let variable simplifies to a constant, references to that variable should simplify to a constant too. For example, in

{$match: {$expr:
  {$let: {
    vars: {upperBound: {$add: [2, 3]}},
    in: {$lt: ["$field", {$add: ["$$upperBound", 10]}]}
  }}
}}

This should simplify down to

{$match: {$expr:
  {$lt: ["$field", 15]}
}}

which thanks to SERVER-39943 can be indexed.

Possibly this is actually 2 separate optimizations:
1. replace each occurrence of "$$upperBound" with 5
2. remove the $let expression since $$upperBound is unused

If some of the vars are constant and some are not, we can apply rule 1 but not rule 2.