PR11326: Make an early check for saved-return-$var validity

PR11326: Make an early check for saved-return-$var validity

We now check validity *before* the entry-probe is generated, so invalid
$vars have a chance to be properly optimized without error.

* tapsets.cxx
  (dwarf_var_expanding_visitor::visit_target_symbol_saved_return):
  Attempt variable expansion immediately, and skip out on failure.
* testsuite/semok/thirtysix.stp: unblock the previously broken part.