Add test for BC_KNIL slot revival under LJ_GC64

Author: corsix

test/trace/gc64_slot_revival.lua

@@ -0,0 +1,7 @@
+do --- BC_KNIL
+  local function f(x, y) end
+  for i = 1,100 do
+    f(i, i)
+    f(nil, nil)
+  end
+end

test/trace/index

@@ -1,6 +1,7 @@
 exit_frame.lua
 exit_growstack.lua
 exit_jfuncf.lua
+gc64_slot_revival.lua
 phi
 snap.lua
 stitch.lua