Add OpSIBLING_nocheck() and use it

This macro is to be used when it is known that things are sane, and
avoids a conditional.  And it avoids warnings that a future commit would
otherwise introduce.

Author: khwilliamson

op.c

@@ -2091,7 +2091,7 @@ Perl_scalar(pTHX_ OP *o)
             kid = cLISTOPo->op_first;
             kid = OpSIBLING(kid); /* get past pushmark */
             assert(OpSIBLING(kid));
-            name = op_varname(OpSIBLING(kid));
+            name = op_varname(OpSIBLING_nocheck(kid));
             if (!name) /* XS module fiddling with the op tree */
                 break;
             warn_elem_scalar_context(kid, name, o->op_type == OP_KVHSLICE, false);

op.h

@@ -1099,7 +1099,8 @@ C<sib> is non-null. For a higher-level interface, see C<L</op_sibling_splice>>.
 #define OP_IS_STAT(op) (OP_IS_FILETEST(op) || (op) == OP_LSTAT || (op) == OP_STAT)
 
 #define OpHAS_SIBLING(o)	(cBOOL((o)->op_moresib))
-#define OpSIBLING(o)		((o)->op_moresib ? (o)->op_sibparent : NULL)
+#define OpSIBLING_nocheck(o)	((o)->op_sibparent)
+#define OpSIBLING(o)		(OpHAS_SIBLING(o) ? OpSIBLING_nocheck(o) : NULL)
 #define OpMORESIB_set(o, sib) ((o)->op_moresib = 1, (o)->op_sibparent = (sib))
 #define OpLASTSIB_set(o, parent) \
     ((o)->op_moresib = 0, (o)->op_sibparent = (parent))

peep.c

@@ -77,7 +77,7 @@ S_scalar_slice_warning(pTHX_ const OP *o)
         return;
 
     assert(OpSIBLING(kid));
-    name = op_varname(OpSIBLING(kid));
+    name = op_varname(OpSIBLING_nocheck(kid));
     if (!name) /* XS module fiddling with the op tree */
         return;
     warn_elem_scalar_context(kid, name, is_hash, true);
@@ -754,7 +754,7 @@ S_maybe_multiconcat(pTHX_ OP *o)
         lastkidop = cLISTOPx(topop)->op_last;
         kid = cUNOPx(topop)->op_first; /* pushmark */
         op_null(kid);
-        op_null(OpSIBLING(kid));       /* const */
+        op_null(OpSIBLING_nocheck(kid));       /* const */
         if (o != topop) {
             kid = op_sibling_splice(topop, NULL, -1, NULL); /* cut all args */
             op_sibling_splice(o, NULL, 0, kid); /* and attach to o */