Model `defer`ed calls so the call runs at function exit rather than inline
at the `defer` statement, reproducing the previous control-flow semantics:
- Add a per-defer "defer-invoke" node for the deferred call.
- deferExitStep wires normal-exit predecessors (return nodes and body
fall-through) through the active deferred-call invocations in
last-in-first-out order, then on to the normal exit target (the
result-read epilogue for named results, or the normal exit node).
- The chain is reachability-gated using the defer-free successor relation
(succIgnoringDeferExit / isInOrderNode), so only deferred calls that were
actually registered on a path are run on that path.
- overridesCallableBodyExit / overridesCallableEndAbruptCompletion suppress
the default body-exit and return routing for functions containing
`defer`, so the epilogue is interposed instead.
Add opt-in InputSig2 predicates to support a function-exit epilogue whose
placement depends on reachability (such as Go's deferred calls, run at
exit in last-in-first-out order):
- deferExitStep: language-provided exit-epilogue edges, wired into
explicitStep but excluded from the defer-free reachability.
- overridesCallableBodyExit: suppresses the default fall-through edge from
a body's "after" node to the normal exit, so the epilogue can be
interposed on fall-through paths.
To let a language compute the reachability gate for those edges without
observing them (and without a non-monotonic cycle through reachable):
- explicitStep is split into explicitStepCommon (defer-free) plus
deferExitStep, and defaultCfg now negates explicitStepCommon so default
evaluation does not depend on deferExitStep.
- succIgnoringDeferExit exposes the defer-free successor relation, typed
over PreControlFlowNode so it does not depend on reachable.
- getASuccessorIgnoringDeferredExit exposes the same relation as a
ControlFlowNode member for general use.
- isInOrderNode exposes a structural, reachability-free node-identity test
for use inside the negations a language needs when computing its gate.
- EntryNodeImpl is no longer private, so a language can identify the entry
node over PreControlFlowNode.
All InputSig2 additions default to none(), leaving other languages
unchanged.
To determine that test9 can't return normally, you have to use the fact
that test5 can't return normally. This would make CFG construction
recursive, which would be bad for performance. Therefore we accept the
limitation that we cannot detect that test9 can't return normally, and
we change the test output.
