gc: fix MEMDEBUG build (#35622)

Sink the assume to the right point, to ensure the compiler doesn't use
this information to mis-optimize the code in MEMDEBUG mode.

Author: vtjnash

src/gc.c

@@ -725,6 +725,7 @@ STATIC_INLINE void gc_setmark_pool_(jl_ptls_t ptls, jl_taggedvalue_t *o,
 #ifdef MEMDEBUG
     gc_setmark_big(ptls, o, mark_mode);
 #else
+    jl_assume(page);
     if (mark_mode == GC_OLD_MARKED) {
         ptls->gc_cache.perm_scanned_bytes += page->osize;
         jl_atomic_fetch_add_relaxed(&page->nold, 1);
@@ -748,7 +749,7 @@ STATIC_INLINE void gc_setmark_pool_(jl_ptls_t ptls, jl_taggedvalue_t *o,
 STATIC_INLINE void gc_setmark_pool(jl_ptls_t ptls, jl_taggedvalue_t *o,
                                    uint8_t mark_mode) JL_NOTSAFEPOINT
 {
-    gc_setmark_pool_(ptls, o, mark_mode, jl_assume(page_metadata(o)));
+    gc_setmark_pool_(ptls, o, mark_mode, page_metadata(o));
 }
 
 STATIC_INLINE void gc_setmark(jl_ptls_t ptls, jl_taggedvalue_t *o,