Allow ternaries in array shape expressions

[This comment](https://github.com/inducer/loopy/issues/229#issuecomment-781443305) shows an example where a, let's say, "odd" array shape results because Loopy picks an overapproximation when determining the shape expression from the access footprint. If we allowed conditionals to exactly round-trip `PwAff`s in shape expresions (or maybe just allow those to directly *be* shape expressions), we wouldn't need to overapproximate.

cc @kaushikcfd