Don't use rule (D) for AstAppend and let simplification pass through

[Mikolaj]

It turns out our handling of AstAppend was broken

https://github.com/Mikolaj/horde-ad/commit/d3ddcf76572f49406a5d729570c6424b88f391c0#diff-89b28a3b50641a4f5bea4f760c491a369d1245c655cb471abec544c7438bc90dR312-R316

and because in our system append can't be expressed as gather nor build, in desperation we now use rule (D) and we don't simplify indexing of append nor fuse gathers from outside and inside of append.

If some tenets of our system need relaxing to accommodate more efficient handling of append, it may be worth it. If rule (D) has to be used nevertheless, we should at least try to simplify more, similarly as AstFromList is simplified maximally despite rule (D) being required for its vectorization.