pp_hot.c: shave off some CPU instructions in pp_const()

CC is MSVC 2022 x64 -O1. Before, 0x67 bytes long. After, 0x5E bytes long.
This savings obtained by not digging inside PL_op AKA my_perl->Iop->op_***
twice, and the CC not having save a C auto var around a function call.
I profiled Perl_pp_const() to be between #2 to #4 most executed pp_*()
function, per process on any workload. For extra details about this commit
see the GH PR associated with this commit.

There is an alternative I privately prototyped but I decided not to
finish 3/4s of the way which is creating pre/post arg eval variants of
function/macro rpp_xpush_1(). It was getting too invasive and complex by
lines of src code modified to finish, so that conceptual fix isn't done
here, and something best left for the future.

Author: bulk88

pp_hot.c

@@ -224,7 +224,9 @@ Perl_rpp_free_2_(pTHX_ SV *const sv1,  SV *const sv2,
 
 PP(pp_const)
 {
-    rpp_xpush_1(cSVOP_sv);
+    rpp_extend(1);      /* Keep together all PL_op derefs (cSVOP_sv/NORMAL) */
+    SV* sv = cSVOP_sv;  /* in this hot pp op. stack_grow() is a function. */
+    rpp_push_1(sv);     /* Previously rpp_xpush_1(). */
     return NORMAL;
 }