pp_split: assign to temp AV in @ary = split(...) optimisation #18090
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The @ary = split(...) optimisation uses SWITCHSTACK to make @ary masquerade as the value stack. However, code that is not aware of this could modify @ary during the split and cause perl to segfault/panic/catch fire. (e.g. see added tests)
This commit makes SWITCHSTACK operate on a temporary AV instead, then swap over the relevant pointers towards the end of the function.
No user-visible changes - besides perl not crashing - are intended.
Note: this PR is standalone but comes from the discussion in #18014