add op_sibling_splice() fn and make core use it

The op_sibling_splice() is a new general-purpose OP manipulation
function designed to edit the children of an op, in an analogous
manner in which the perl splice() function manipulates arrays.

This commit also edits op.c and a few other places to remove most direct
manipulation of op_sibling, op_first and op_last, and replace that with
calls to op_sibling_splice().

This has two advantages. First, by using the one function consistently
throughout, it makes it clearer what a particular piece of of code is
doing, rather than having to decipher lots of of ad-hoc

    cLISTOPo->op_first = OP_SIBLING(kid);

style stuff. Second, it will make it easier to later add a facility for
child OPs to find their parent, since the changes now only need to be made
in a few places.

In theory this commit should make no functional change to the code.

1 files changed, 5 insertions, 0 deletions

diff --git a/proto.h b/proto.h
index 46c41bce96..de11c62b3f 100644
--- a/proto.h
+++ b/proto.h
@@ -3125,6 +3125,11 @@ PERL_CALLCONV OP*	Perl_op_prepend_elem(pTHX_ I32 optype, OP* first, OP* last);
 PERL_CALLCONV void	Perl_op_refcnt_lock(pTHX);
 PERL_CALLCONV void	Perl_op_refcnt_unlock(pTHX);
 PERL_CALLCONV OP*	Perl_op_scope(pTHX_ OP* o);
+PERL_CALLCONV OP*	Perl_op_sibling_splice(pTHX_ OP *parent, OP *start, int del_count, OP* insert)
+			__attribute__nonnull__(pTHX_1);
+#define PERL_ARGS_ASSERT_OP_SIBLING_SPLICE	\
+	assert(parent)
+
 PERL_CALLCONV OP*	Perl_op_unscope(pTHX_ OP* o);
 PERL_CALLCONV void	Perl_pack_cat(pTHX_ SV *cat, const char *pat, const char *patend, SV **beglist, SV **endlist, SV ***next_in_list, U32 flags)
 			__attribute__nonnull__(pTHX_1)