sv_upgrade: c99 compound literals to initialize xpvav/xpvhv

For both the AV and HV cases, the new body is Zero()ed but then various
struct members are set to non-zero values. Now that we support parts of
c99, it seems more efficient to use compound literals to initailize the
struct members.

With gcc v8.3.0, the compiled function is smaller by 25 instructions.
sv.o is slightly smaller, but the final perl binary size is unchanged.

Author: richardleach

sv.c

@@ -1359,26 +1359,34 @@ Perl_sv_upgrade(pTHX_ SV *const sv, svtype new_type)
         assert(new_type_details->arena_size);
         /* This points to the start of the allocated area.  */
         new_body = S_new_body(aTHX_ new_type);
-        Zero(new_body, new_type_details->body_size, char);
-        new_body = ((char *)new_body) - new_type_details->offset;
+        /* xpvav and xpvhv have no offset, so no need to adjust new_body */
+        assert(!(new_type_details->offset));
 #else
         /* We always allocated the full length item with PURIFY. To do this
            we fake things so that arena is false for all 16 types..  */
         new_body = new_NOARENAZ(new_type_details);
 #endif
         SvANY(sv) = new_body;
         if (new_type == SVt_PVAV) {
-            AvMAX(sv)	= -1;
-            AvFILLp(sv)	= -1;
+            *((XPVAV*) SvANY(sv)) = (XPVAV) {
+                .xmg_stash = NULL, .xmg_u = {.xmg_magic = NULL},
+                .xav_fill = -1, .xav_max = -1, .xav_alloc = 0
+                };
+
             AvREAL_only(sv);
         } else {
+            *((XPVHV*) SvANY(sv)) = (XPVHV) {
+                .xmg_stash = NULL, .xmg_u = {.xmg_magic = NULL},
+                .xhv_keys = 0,
+                /* start with PERL_HASH_DEFAULT_HvMAX+1 buckets: */
+                .xhv_max = PERL_HASH_DEFAULT_HvMAX
+                };
+
             assert(!SvOK(sv));
             SvOK_off(sv);
 #ifndef NODEFAULT_SHAREKEYS
             HvSHAREKEYS_on(sv);         /* key-sharing on by default */
 #endif
-            /* start with PERL_HASH_DEFAULT_HvMAX+1 buckets: */
-            HvMAX(sv) = PERL_HASH_DEFAULT_HvMAX;
         }
 
         /* SVt_NULL isn't the only thing upgraded to AV or HV.