swift

eeeeee24 - [AutoDiff] Remove differentiation order from AD-related instructions. (#27579)

[AutoDiff] Remove differentiation order from AD-related instructions. (#27579) The differentiation order field in `differentiable_function` and `differentiable_function_extract` instructions is unsupported and will not be used by the current design. Quite a lot of dead code exists to try to handle `order`, but it is mostly incomplete and untested. This PR removes the differentiation order from the code base to simplify what we upstream to the 'master' branch. Changes include: * Remove `differentiationOrder` from `DifferentiableFunctionInst` and `DifferentiableFunctionExtractInst`. * Make `DifferentiableFunctionInst::DifferentiableFunctionInst` take an optional pair of JVP and VJP instead of a variable-size array. * Rename "associated functions" to "derivative functions" in `DifferentiableFunctionInst` to align better with [the design](https://forums.swift.org/t/differentiable-programming-mega-proposal/28547). Filed task [TF-882](https://bugs.swift.org/browse/TF-882) to track the renaming of all other occurrences of "associated functions". Resolves [TF-880](https://bugs.swift.org/browse/TF-880).