Report parent of stitched trace.

Thanks to Nick Zavaritsky.

src/jit/dump.lua

@@ -556,7 +556,7 @@ local function dump_trace(what, tr, func, pc, otr, oex)
   if what == "start" then
     if dumpmode.H then out:write('<pre class="ljdump">\n') end
     out:write("---- TRACE ", tr, " ", what)
-    if otr then out:write(" ", otr, "/", oex) end
+    if otr then out:write(" ", otr, "/", oex == -1 and "stitch" or oex) end
     out:write(" ", fmtfunc(func, pc), "\n")
   elseif what == "stop" or what == "abort" then
     out:write("---- TRACE ", tr, " ", what)