I don't see any way to get rid of this variable without adding a new split-type flagging the fact we're not discarding the original page, and then doing more work inside of discard for that case.   (And, that solution would require we know something extra about this case, we'd have to have a line that does something like `page->ref = &mod->u.split->u.intl.t[0]` instead of the WT_LINK_PAGE line in rec_merge.c.)

One cleanup would be to always set & use the split_parent_ref field, that is, it's always set in the case of a split page -- I think that change is pretty straight-forward and would make this less of a special case.

Also, we should add this field into the union, because the existing WT_ADDR structure is bigger than a single pointer, so we'll save 8B in  the page modify structure.  Something like:

```
union {
        struct {
                WT_PAGE *page;    /* Resulting split page */
                WT_REF *ref;      /* WT_REF slot on the parent */
        } split;
        WT_ADDR    replace;      /* Resulting replacement */
} u;
```