Optimise away LIST/ex-LIST trees containing one item

[richardleach]

Description
In scalar context, a (possibly-nulled) LIST OP may effectively contain a single item.
It seems like this container OP and redundant KID OPs could be removed entirely.

For example, my $x = (1,2,3) leaves Perl_scalar as:

2     <;> nextstate(main 2 -e:1) v:%,us,{,fea=15 ->3
7     <2> sassign vKS/2 ->8
5        <@> list sKP ->6
3           <0> pushmark v ->4
-           <0> ex-const v ->-
-           <0> ex-const v ->4
4           <$> const[IV 3] s ->5
6        <0> padsv[$x:2,3] sRM*/LVINTRO ->7

Could this be optimised to:

2     <;> nextstate(main 2 -e:1) v:%,us,{,fea=15 ->3
5     <2> sassign vKS/2 ->6
3        <$> const[IV 3] s ->4
4        <0> padsv[$x:2,3] sRM*/LVINTRO ->5

See also #23447.

Steps to Reproduce
perl -MO=Concise -E 'my $x = (1,2,3)'

Perl configuration
5.43.0 and previous.