JIT: We should try to maintain full defs through async liveness

When we decompose stores physical promotion will often end up introducing STORE_LCL_FLDs if the destination was not fully promoted. Unfortunately these often result in async liveness thinking that the stores are partial, even though the full set is actually a full definition. This can then lead to unnecessary state being retained.

To fixes I can think of:
- Try to make async liveness more precise and track per-field state
- Instead of creating a set of `STORE_LCL_FLD` on decomposition, instead create `STORE_LCL_VAR(FIELD_LIST(...))` and then expand it in lowering.

I am partial to (2), it seems simpler.

No files or tests are named. Start by tracing JIT store decomposition, async liveness, and lowering to determine where STORE_LCL_FLDs are produced and interpreted; done means full definitions remain recognized as full and unnecessary state is no longer retained.

Written by the indexing model from the issue text.