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_ir.h

@@ -317,7 +317,7 @@ typedef enum {
   IRT_THREAD,
   IRT_PROTO,
   IRT_FUNC,
-  IRT_9,		/* LJ_TDEADKEY is never used in the IR. */
+  IRT_9,		/* Never used in the IR. */
   IRT_TAB,
   IRT_UDATA,
   /* ... until here. */