Make OP_DEFINED eligible for constant folding

[richardleach]

Prior to this, the condition in the following example did not fold:

'use constant ONE => 1; my $x = (defined(ONE)) ? 1 : 0;'

With this commit, the final op tree produced for the above is just:

5  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter v ->2
2     <;> nextstate(main 193 -e:1) v:%,us,{,fea=15 ->3
4     <1> padsv_store[$x:193,194] vKS/LVINTRO ->5
3        <$> const[IV 1] s/FOLD ->4
-        <0> ex-padsv sRM*/LVINTRO ->4

Closes #16266

---

• This seems like a bit of an edge case, so I don't think it needs a perldelta entry.

[tonycoz]

The op tree in the commit message doesn't match the perl code (where did $x come from?)

[richardleach]

The op tree in the commit message doesn't match the perl code (where did $x come from?)

Sorry about that. I changed examples a couple of times while writing. Now updated to match.

[richardleach]

@tonycoz - am I okay to merge this?