lj_jit.h: Save 'parent' and 'exitno' fields in GCtrace

For debugging purposes it is very useful to be able to refer to the
origin of a trace (parent/exit) and so this change stores that
information persistently in GCtrace instead of only ephemerally in
jit_State.

These fields are now duplicated in jit_State (valid while recording)
and GCtrace (valid after recording.) This duplication could be avoided
by putting them only in GCtrace and accessing them via J->cur but this
would be a more noisy change since the existing fields are accessed
from many places including DynASM macros.

src/lj_jit.h

@@ -199,6 +199,8 @@ typedef struct GCtrace {
   TraceNo1 root;	/* Root trace of side trace (or 0 for root traces). */
   TraceNo1 nextroot;	/* Next root trace for same prototype. */
   TraceNo1 nextside;	/* Next side trace of same root trace. */
+  TraceNo1 parent;      /* Parent of this trace (or 0 for root traces). */
+  ExitNo exitno;        /* Exit number in parent (valid for side-traces only). */
   uint8_t sinktags;	/* Trace has SINK tags. */
   uint8_t unused1;
 } GCtrace;

src/lj_trace.c

@@ -109,6 +109,8 @@ static void trace_save(jit_State *J, GCtrace *T)
   size_t szins = (J->cur.nins-J->cur.nk)*sizeof(IRIns);
   char *p = (char *)T + sztr;
   memcpy(T, &J->cur, sizeof(GCtrace));
+  T->parent = J->parent;
+  T->exitno = J->exitno;
   setgcrefr(T->nextgc, J2G(J)->gc.root);
   setgcrefp(J2G(J)->gc.root, T);
   newwhite(J2G(J), T);