`Bug` Quadratic(?) compiler perf for huge nested expressions

While investigating #12420 I noticed that one particular part of our optimizer seems quadratic for very large nested expressions made of nothing but function/method applications, e.g. `M(M(M(M.....))))` and the kind of code that comes out of computation expressions.

The problem is already noted in our code, see [permalink](https://github.com/dotnet/fsharp/blob/f0f9c17aa45ca963bd8bf69ce7c72883e4dafc3f/src/fsharp/Optimizer.fs#L2541-L2543)

This isn't critical but we should fix this sometime. It appears to be re-traversing expressions determining free variables.