selftests/bpf: Tests for SCEV analysis and loop widening

- Computed expression shapes: register and spilled-counter recurrences,
  substitution at latches, agreeing and conflicting backedge updates,
  and ALU, extension, byte-swap, load/store and atomic expressions.

- Iteration bounds and header-visit counts for:
  - pre-condition loops using JEQ, JLT, JLE, JGE and JGT;
  - post-condition loops using JLT, JLE, JGE, JGT and JNE;
  - signed pre- and post-conditions using JSLT, JSLE, JSGT and JSGE;
  - increasing and decreasing counters, nonzero initial values,
    immediate and invariant-register bounds, and immediate exits;
  - additional exits that reduce the minimum iteration count or
    tighten the upper bound;
  - rounded bounds, counter wraparound, count overflow, extreme
    steps and JNE divisibility.

- Widening, backedge clamping and state-pruning convergence,
  including multiple induction variables used for map-value accesses
  and nonconstant entry values requiring a common alignment.
  Check precision propagation to initial values and bounds.

- Nested and sibling loops, shared headers, exits to another loop
  header, exits across multiple nesting levels, and side entries
  into loop nests.
  Check conservative summaries for irreducible children while still
  widening supported reducible descendants, value preservation on
  inner-loop exits, and detection of nonterminating outer loops.

- Invalidation of stack-slot expressions after indirect stack writes
  and global calls. Widening across bpf_probe_read_kernel() calls
  in static subprograms.

- Stack spill widths and slot states, including narrow spills, zero,
  unallocated and STACK_MISC slots, and counters spilled and filled
  using store-release/load-acquire.

- Avoid widening registers used to address stack spills, fills and
  dynptr constructor arguments, including dependencies through nested
  loops. Conversely, allow widening addresses of byte-sized stores.

- Conditional assignments joining scalar values and stack pointers,
  including preservation of a common base and gcd-derived step.

- Pointer widening and comparisons with compatible memory origins,
  fallback for incompatible origins and nullable pointers, and
  invalidation of stale packet pointer relationships.

- Conservative fallback for unsupported recurrences, unavailable
  counter spills and zero computed header counts.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261009-scev-minimal-rebase-v3-51-bcba9bc53ac9@gmail.com

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