[imcc] Fix most -O2 errors, restore ins2->op on failed const propag.

IMCC_subst_constants may return ok with an empty ins.
Restore the old ins2->op ptr then.
If !ok also restore the symregs.

Remaining -O2 fails:
/compilers/imcc/syn/macro.t      (Wstat: 512 Tests: 42 Failed: 2)
  Failed tests:  10-11
  Non-zero exit status: 2
t/op/calling.t                    (Wstat: 512 Tests: 104 Failed: 2)
  Failed tests:  30, 36
  Non-zero exit status: 2
t/op/exit.t                       (Wstat: 256 Tests: 9 Failed: 1)
  Failed test:  6
  Non-zero exit status: 1
t/op/jitn.t                       (Wstat: 256 Tests: 14 Failed: 1)
  Failed test:  13
  Non-zero exit status: 1
t/op/lexicals.t                   (Wstat: 256 Tests: 54 Failed: 1)
  Failed test:  42
  Non-zero exit status: 1
t/pmc/io.t                        (Wstat: 1024 Tests: 35 Failed: 4)
  Failed tests:  3-4, 13-14
  Non-zero exit status: 4
t/pmc/nci.t                       (Wstat: 256 Tests: 61 Failed: 1)
  Failed test:  50
  Non-zero exit status: 1
t/pmc/os.t                        (Wstat: 512 Tests: 36 Failed: 2)
  Failed tests:  3, 5
  Non-zero exit status: 2
Files=207, Tests=7199, 72 wallclock secs ( 1.46 usr  0.49 sys + 32.52 cusr 24.70 csys = 59.17 CPU)
Result: FAIL