No longer let the GC replace dead keys with the LJ_TDEADKEY tag.

Important: this changes the semantics of the write barrier!
Carefully read the big comment block in lj_obj.h
This helps HREFK key slot specialization and allows safely hoisting
HREF/HREFK across GC steps, too (fix for a barely reproducible bug).
Dead keys are only removed during a table resize (as before).

src/lj_lib.c

@@ -53,7 +53,7 @@ void lj_lib_register(lua_State *L, const char *libname,
   ptrdiff_t tpos = L->top - L->base;
 
   /* Avoid barriers further down. */
-  if (isblack(obj2gco(tab))) lj_gc_barrierback(G(L), tab);
+  lj_gc_anybarriert(L, tab);
   tab->nomm = 0;
 
   for (;;) {