Simplify GCtrace * reference embedding for trace stitching.

This is now possible due to the immovable IR.
Contributed by Peter Cawley.

src/lj_jit.h

@@ -416,7 +416,7 @@ typedef struct jit_State {
   GCRef *trace;		/* Array of traces. */
   TraceNo freetrace;	/* Start of scan for next free trace. */
   MSize sizetrace;	/* Size of trace array. */
-  TValue *ktracep;	/* Pointer to K64Array slot with GCtrace pointer. */
+  IRRef1 ktrace;	/* Reference to KGC with GCtrace. */
 
   IRRef1 chain[IR__MAX];  /* IR instruction skip-list chain anchors. */
   TRef slot[LJ_MAX_JSLOTS+LJ_STACK_EXTRA];  /* Stack slot map. */