Peephole optimizations for NULLs in lists and scalar lists

[richardleach]

my $x = (1,2,3); produces the following OP tree in blead:

2     <;> nextstate(main 1 -e:1) v:{ ->3
6     <1> padsv_store[$x:1,2] vKS/LVINTRO ->7
5        <@> list sKP ->6
3           <0> pushmark v ->4
-           <0> ex-const v ->-
-           <0> ex-const v ->4
4           <$> const(IV 3) s ->5
-        <0> ex-padsv sRM*/LVINTRO ->6

This is functionally equivalent to my $x = 3;:

2     <;> nextstate(main 1 -e:1) v:{ ->3
4     <1> padsv_store[$x:1,2] vKS/LVINTRO ->5
3        <$> const(IV 3) s ->4
-        <0> ex-padsv sRM*/LVINTRO ->4

Refactoring Perl_scalar / Perl_scalarvoid to allow the transformation of the first tree into the second is proabably not worth the churn, given that constructing the first tree will typically emit "Useless use of X in scalar context" warnings. Some cases may silently slip through though, such as when the consts 0 and 1 are excluded from these warnings.

The attached commits enable the peephole optimizer to do two things:

1. Remove OP_NULL list nodes, when they have no kids and have a sibling.
2. Adjust the op_next pointers to skip over the unnecessary OP_PUSHMARK and OP_LIST nodes.

That gives an OP tree like this:

2     <;> nextstate(main 1 -e:1) v:{ ->3
4     <1> padsv_store[$x:1,2] vKS/LVINTRO ->5
-        <1> ex-list sKP ->4
-           <0> ex-pushmark v ->3
3           <$> const[IV 3] s ->4
-        <0> ex-padsv sRM*/LVINTRO ->4

Closes: #23448

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