Evaluate expressions after type coercion

[Dandandan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The recently added an optimizer pass that performs type coercion on the logical plan.
There is some oppurtunity to simplify the expressions as often it creates casts on constants, like:

CAST(Int64(1) AS Float32)

This could be evaluated to:
Float32(1.0)

Simplifying the plan and leaving more room for further optimization.

Describe the solution you'd like
When wrapping the expression in a cast, we can directly evaluate the result. Alternatively we could run the "simplify_expressions" pass again after type coercion, however this seems unneeded.
It seems we can use the evaluate_to_scalar function / ConstEvaluator to evaluate the created Expr in the type coercion, after applying it to an expression.
https://github.com/apache/arrow-datafusion/blob/master/datafusion/optimizer/src/simplify_expressions.rs#L487

Describe alternatives you've considered
n/a

Additional context