gh-126835: Fold unary & binary complex constant expressions during code generation.

[WolframAlph]

Codegen expects unary & binary complex constant expressions already folded. Without it, we cannot proceed with migration of unaryop & binaryop to CFG. cc @Eclips4 @tomasr8

This PR does 2 things:

1. Implement unary & binary complex constant expression folding in codegen to prepare for binary & unary folding migration.
2. Make AST optimization optional, to test mentioned changes.

This PR is created in case we decide to go with this approach.

• Issue: Move const folding to the peephole optimizer #126835

[WolframAlph]

Used different approach in the end.