Auto merge of #139281 - petrochenkov:ctxtdecod6, r=wesleywiser

hygiene: Avoid recursion in syntax context decoding

#139241 has two components
- Avoiding recursion during syntax context decoding
- Encoding/decoding only the non-redundant data, and recalculating the redundant data again during decoding

Both of these parts may influence compilation times, possibly in opposite directions.
So this PR contains only the first part to evaluate its effect in isolation.

Author: bors