pp.c pp_argelem() dont make SV* tmp bufs in a loop to strip GMG

[bulk88]

Alloc the mortal string scratch pad SV* only once and reuse it for every iteration to strip GMG from each SV* key. hv_store_ent() never RC++s ownership of the callers SV* key arg.

Although it makes no difference either way for pp_argelem(), because it is an internal detail of sv.c Use any type of PV COW if available. If a shared HEK COW is delivered into the tmp SV*, even better b/c of hv_store_ent()'s optimization for that.

Worst case keynomg has a Newx() buffer that only reallocs 1x or 2x before flatlining for the rest of the loop which is still very good.

I think, but I can't confirm, that this copy code was added b/c of a GMG getter firing order issue so val doesn't GMG b4 key, or because maybe its not possible to prevent hv_common() keysv GMG from firing so SvGETMAGIC() can't be used to reverse GMG firing order, or maybe if a keysv GMG getter throws an exception inside hv_common(), tmpsv is leaked.

The make multiple mortal SV PVs in a loop code is from day 1 of this opcode in commit 4fa0684 - davem - 7/9/2016 5:41:08 AM

"add OP_ARGELEM, OP_ARGDEFELEM, OP_ARGCHECK ops"

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• This set of changes does not require a perldelta entry.