OP_SUBSTR_LEFT: GH#22914 - multiple pointers to replacement OP

[richardleach]

The recent initial commit for OP_SUBSTR_LEFT failed to account for
there being multiple paths from a non-trivial LENGTH to the ""
replacement CONST OP. This could result in the replacement SV
being erroneously pushed to the stack, causing pp_substr_left
to try to operate on the wrong SV.

This commit nulls out the replacement OP, so that even if it
is encountered, no erroneous SV is pushed. Contrary to the
comment in the original commit, this actually does not break
B::Deparse.

Thanks to @mauke for figuring this out and preparing a patch
before I'd even opened my browser.

---

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

[richardleach]

I'll look over this with fresh eyes in the morning, seemed worth opening this PR now for overnight CI etc.

[jkeenan]

I created a local branch from this p.r., then built an unthreaded build on Linux, ran the test suite and attempted to install the 3 CPAN distros identified so far as failing. Results good.

[gh-22918-richardleach-hydahy-substr_left_null_repl-20250115] 2102 $ ./bin/perl -Ilib -MIO::Async -MSpreadsheet::WriteExcel -MRDF::Trine -E 'say q|hello world|;'
hello world

Considerations:

1. I suspect there will be addition CPAN breakage reported against cdbed2a.
2. I haven't examined the code or test suite changes.

[richardleach]

I created a local branch from this p.r., then built an unthreaded build on Linux, ran the test suite and attempted to install the 3 CPAN distros identified so far as failing. Results good.

Thanks for the prompt testing, @jkeenan.

While we may want to improve on the commit contents (e.g. tests) down the line, the better handling of multiple LENGTH pointers is clearly desirable. In the interest of reducing the number of BBC tickets coming in ahead of the next point release, I'm going to merge now and will keep an eye on incoming new issues.

[leonerd]

Happy to confirm that this also fixes a recent IO::Async breakage:
http://www.cpantesters.org/cpan/report/e21668de-d265-11ef-b171-a1011627361f