[Bug other/68874] Allow pass groups to be cloned

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68874

--- Comment #1 from vries at gcc dot gnu.org ---
Created attachment 37009
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37009&action=edit
tentative patch

This tentative patch introduces a prev variable (maintained in NEXT_PASS) and
uses that to get to the previous pass instance in PUSH_INSERT_PASSES_WITHIN.

Note that this ignores the pass argument to PUSH_INSERT_PASSES_WITHIN, so we
could go to the point of removing it:
...
  NEXT_PASS (pass_build_ssa_passes);
  PUSH_INSERT_PASSES_WITHIN ()
      NEXT_PASS (pass_fixup_cfg);
      NEXT_PASS (pass_init_datastructures);
      NEXT_PASS (pass_build_ssa);
  POP_INSERT_PASSES ()
  ...
  NEXT_PASS (pass_build_ssa_passes);
  PUSH_INSERT_PASSES_WITHIN ()
      NEXT_PASS (pass_ubsan);
      NEXT_PASS (pass_early_warn_uninitialized);
      NEXT_PASS (pass_nothrow);
  POP_INSERT_PASSES ()
...