Tweak autocast-defined interactions further

Tweak autocast-defined interactions further

- collapse basic @defined($foo) right away.
- last-ditch collapse other @defined(expr) to 1 or 0 depending on pe_unknown.
- run that last-ditch effort *before* turning on assert_resolvability.
- only run extra dead_control_remover for optimized runs
- in var_expanding_visitor, pass *any* unchanged expr through, so they
  may be decided later.  (e.g. for @choose_defined ternaries)