add rpp_replace_at_norc() to API

It's like rpp_replace_at(), but it assumes that the SV has already had
its refcount bumped. It replaces ugly code like

        SV *nsv = newSVsv(*svp);
    #ifdef PERL_RC_STACK
        rpp_replace(svp, nsv);
        SvREFCNT_dec(nsv);
    #else
        rpp_replace(svp, sv_2mortal(nsv));
    #endif

with a simple

    rpp_replace_at_norc(svp, newSvsv(*svp));

On PERL_RC_STACK builds it's more efficient than before as it's not
unnecessarily bumping and then immediately unbumping nsv's refcount.

embed.fnc

@@ -2774,6 +2774,9 @@ Adipx	|void	|rpp_replace_1_1|NN SV *sv
 Adipx	|void	|rpp_replace_2_1|NN SV *sv
 Adipx	|void	|rpp_replace_at |NN SV **sp				\
 				|NN SV *sv
+Adipx	|void	|rpp_replace_at_norc					\
+				|NN SV **sp				\
+				|NN SV *sv
 Adipx	|bool	|rpp_stack_is_rc
 Adipx	|bool	|rpp_try_AMAGIC_1					\
 				|int method				\

embed.h

@@ -552,6 +552,7 @@
 # define rpp_replace_1_1(a)                     Perl_rpp_replace_1_1(aTHX_ a)
 # define rpp_replace_2_1(a)                     Perl_rpp_replace_2_1(aTHX_ a)
 # define rpp_replace_at(a,b)                    Perl_rpp_replace_at(aTHX_ a,b)
+# define rpp_replace_at_norc(a,b)               Perl_rpp_replace_at_norc(aTHX_ a,b)
 # define rpp_stack_is_rc()                      Perl_rpp_stack_is_rc(aTHX)
 # define rpp_try_AMAGIC_1(a,b)                  Perl_rpp_try_AMAGIC_1(aTHX_ a,b)
 # define rpp_try_AMAGIC_2(a,b)                  Perl_rpp_try_AMAGIC_2(aTHX_ a,b)

inline.h

@@ -705,6 +705,39 @@ Perl_rpp_replace_at(pTHX_ SV **sp, SV *sv)
 }
 
 
+/*
+=for apidoc rpp_replace_at_norc
+
+Replace the SV at address sp within the stack with C<sv>, while suitably
+adjusting the reference count of the old SV. Equivalent to C<*sp = sv>,
+except with proper reference count handling.
+
+C<sv>'s reference count doesn't get incremented. On non-C<PERL_RC_STACK>
+builds, it gets mortalised too.
+
+This is most useful where an SV has just been created and already has a
+reference count of 1, but has not yet been anchored anywhere.
+
+=cut
+*/
+
+PERL_STATIC_INLINE void
+Perl_rpp_replace_at_norc(pTHX_ SV **sp, SV *sv)
+{
+    PERL_ARGS_ASSERT_RPP_REPLACE_AT_NORC;
+
+#ifdef PERL_RC_STACK
+    assert(rpp_stack_is_rc());
+    SV *oldsv = *sp;
+    *sp = sv;
+    SvREFCNT_dec(oldsv);
+#else
+    *sp = sv;
+    sv_2mortal(sv);
+#endif
+}
+
+
 /*
 =for apidoc rpp_context
 

pod/perlguts.pod

@@ -4760,6 +4760,7 @@ in summary:
  rpp_replace_1_1(sv)     (void)POPs; PUSHs(sv);
  rpp_replace_2_1(sv)     (void)POPs; (void)POPs; PUSHs(sv);
  rpp_replace_at(sp, sv)  *sp = sv;
+ rpp_replace_at_norc(sp, sv) *sp = sv_2mortal(sv);
 
  rpp_context(mark, gimme,
              extra)      SP -= extra;
@@ -4809,6 +4810,11 @@ handle edge cases such as an old and new SV being the same.
 rpp_replace_at(sp, sv) is similar to rpp_replace_1_1(), except that
 it replaces an SV at an address in the stack rather than at the top.
 
+rpp_replace_at_norc(sp, sv) is similar to rpp_replace_at(), except that
+it assumes that C<sv> already has a bumped reference count. So, a bit
+like rpp_push_1_norc() (see below), it doesn't bother increasing C<sv>'s
+reference count, or on non-RC builds it mortalises it instead.
+
 rpp_popfree_to(svp) is designed to replace code like
 
     PL_stack_sp = PL_stack_base + cx->blk_oldsp;