Prune statements after a return at -O0

Split from [[feature-a-fold-the-consensus-dead-branch-core-at-every-level]], whose parts 1 (if/while), 2 (escape guard), 3 (-OO) and 4 (charter) are done. This is what its part 1 listed third and nothing has closed.

Measured, 2026-09-02, at the tree that landed the if/while prune

procedure P; begin Exit; WriteLn(NeverR); end;   { never_r_P declared, undefined }

-O0   rc=127  undefined symbol: never_r_P
-O2   alive
-O3   alive

IROptDeadCode catches it from -O1 up. -O0 is the only failing level.

Why it was NOT bolted onto the prune that landed

if False and while False are one question — is this condition a constant — answered by one helper (ASTConstCond) at two sites. This is a different question: which statements in a sequence are still reachable, which needs a notion of which node kinds TERMINATE a block (Exit, return, goto, Halt, a raise, a call to a noreturn), applied while walking AN_SEQ. Bolting it onto a constant-condition fold would have been the second path that stays broken (normalise-dont-special-case).

Constraints inherited from the parent, both non-negotiable

• The label-escape guard applies unchanged. A statement after a return that carries AN_LABEL / AN_LABELADDR / AN_GOTO_INDIRECT is reachable by goto and must be KEPT — this is how goto-based cleanup epilogues are written, so it is the common case in C, not a corner.
• -OO must keep emitting it. The gate is SourceOneToOne, checked at the helper rather than the call site, so a new fold inherits it automatically.

Acceptance

• The shape above links and runs at every level, -O0 included.
• A label after a return still resolves at every level.
• -OO still emits it (assert the FAILURE, as test_source_one_to_one_oo.pas does — a flag that silently did nothing would pass a success assertion).
• Isolating pre/post differential over the corpus: image identity at -O2 (the existing pass already reached this fixed point, so the new lowering must agree with it, not compete), output identity at -O0.

Log

• 2026-09-02 — resolved; this names the commit that carried the resolve, which is not always the one that carried the change — commit ef96c4c18.