Experiment: annotate hot paths on LLVM

[jiribenes]

My local devenv is broken so I can't benchmark this; we should really benchmark each commit individually against main to see if we get some perf.

There are three main changes:

1. assume RC ≥ 0 (not obvious to LLVM since we use a whole i64) EDIT: removed this via a force push for now, diff here
2. when switching on RC, assume that it's 0 for the hot path to be quicker
3. assume that a to-be-erased value is non-null EDIT: not true, we even emit them null sometimes... diff here
4. dumping a bunch of "this function doesn't do bad stuff" annotations all over the place

I think these are all valid assumptions / hot paths that we should prefer.
EDIT: The crossed out changes above kept the CI in a busy loop somehow 🔁