Merge branch 'Add precision propagation for subprogs and callbacks'

Andrii Nakryiko says:

====================
As more and more real-world BPF programs become more complex
and increasingly use subprograms (both static and global), scalar precision
tracking and its (previously weak) support for BPF subprograms (and callbacks
as a special case of that) is becoming more and more of an issue and
limitation. Couple that with increasing reliance on state equivalence (BPF
open-coded iterators have a hard requirement for state equivalence to converge
and successfully validate loops), and it becomes pretty critical to address
this limitation and make precision tracking universally supported for BPF
programs of any complexity and composition.

This patch set teaches BPF verifier to support SCALAR precision
backpropagation across multiple frames (for subprogram calls and callback
simulations) and addresses most practical situations (SCALAR stack
loads/stores using registers other than r10 being the last remaining
limitation, though thankfully rarely used in practice).

Main logic is explained in details in patch #8. The rest are preliminary
preparations, refactorings, clean ups, and fixes. See respective patches for
details.

Patch #8 has also veristat comparison of results for selftests, Cilium, and
some of Meta production BPF programs before and after these changes.

v2->v3:
  - drop bitcnt and ifs from bt_xxx() helpers (Alexei);
v1->v2:
  - addressed review feedback form Alexei, adjusted commit messages, comments,
    added verbose(), WARN_ONCE(), etc;
  - re-ran all the tests and veristat on selftests, cilium, and meta-internal
    code: no new changes and no kernel warnings.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>