C inc-i340: fix -fstash regressions but still not good enough

all 5.14 tests pass now, 5.18 200 also,
but we got many testcore regressions:
t/C-COMPILED/CORE--base--lex.t                         (Wstat: 1536 Tests: 23 Failed: 6)
  Failed tests:  8-9, 12, 18-19, 22
t/C-COMPILED/CORE--mro--next_edgecases.t               (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--op--closure.t                       (Wstat: 1024 Tests: 23 Failed: 4)
  Failed tests:  10-11, 20-21
t/C-COMPILED/CORE--op--dbm.t                           (Wstat: 1024 Tests: 23 Failed: 4)
  Failed tests:  10-11, 20-21
t/C-COMPILED/CORE--op--inccode.t                       (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--op--length.t                        (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--op--sigdispatch.t                   (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--op--undef.t                         (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--re--charset.t                       (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22
t/C-COMPILED/CORE--re--substr.t                        (Wstat: 768 Tests: 23 Failed: 3)
  Failed tests:  17-18, 22