Currently the UseVarIfNotSimple method returns `null` for the binding if the value is simple, and later `AstExpression.Let` has special logic for skipping any binding that is `null`.

It would be simpler and generalize better if we moved the simplification of `$let` to the `AstSimplifier`. The simplification consists of removing any bindings where the value is "simple" and replacing var references with the simple expression. In many cases this will mean that the `$let` can be entirely optimized away. And even when not, it will likely result in fewer variables being needed.