Optimize Winterfell codegen in some expressions

Following https://github.com/0xPolygonMiden/air-script/pull/352, the codegen could be improved in some instance, for example when we reduce exponentiations into multiplications.

- We can introduce temporary variables to hold results of common sub-expressions (e.g. powers of two of exponentiated elements)
- We can factorize extension field conversion in expressions that contain multiple instances of it

Additional context in the originally comment posted by @bobbinth in https://github.com/0xPolygonMiden/air-script/pull/352#discussion_r1711059613_