peep.c: Only remove expected empty if/else block structures

[richardleach]

#23367 expected that empty OP_COND_EXPR
branches would produce one of the following OP tree structures fit for removal:

  +-- stub

or

  +-- scope
    |
    +-- stub

GH #23505 showed that { BEGIN {} } blocks will also generate the likes of:

  +--scope
    |
    +--stub
    |
    +--null (ex-nextstate)

This commit does the minimal possible to fix this bug - the peephole optimizer
now checks for the expected cases and ignores anything else.

Future commits may extend the optimization to remove this additional type of
structure. Additional tests have been added now and give a useful structure for
future commit tests to fit into.

---

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

[richardleach]

Rather than handling the BEGIN {} case in the peephole optimizer, it seems tidier to handle it back in Perl_op_scope, ensuring that one of the expected OP_STUB patterns results.

However, initial work showed that Perl_pmruntime and S_regmatch (case EVAL:) would have to be amended to recognise bare OP_STUBs. (It would be nice if S_regmatch could be made to not execute OP trees that are functionally no-ops.)

I'll work on this a bit more in the background.