fixes a serious codegen bug that caused to emit wrong barriers in rare cases

tests/parallel/twrong_refcounts.nim

@@ -0,0 +1,53 @@
+discard """
+  output: "Success"
+"""
+
+import math, threadPool
+
+# ---
+
+type
+  Person = object
+    age: int
+    friend: ref Person
+
+var
+  people: seq[ref Person] = @[]
+
+proc newPerson(age:int): ref Person =
+  result.new()
+  result.age = age
+
+proc greet(p:Person) =
+  #echo p.age, ", ", p.friend.age
+  p.friend.age += 1
+
+# ---
+
+proc setup =
+  for i in 0 .. <20:
+    people.add newPerson(i + 1)
+  for i in 0 .. <20:
+    people[i].friend = people[random(20)]
+
+proc update =
+  var countA: array[20, int]
+  var countB: array[20, int]
+
+  for i, p in people:
+    countA[i] = getRefCount(p)
+  parallel:
+    for i in 0 .. people.high:
+      spawn greet(people[i][])
+  for i, p in people:
+    countB[i] = getRefCount(p)
+
+  for i in 0 .. <20:
+    doAssert countA[i] == countB[i]
+  echo "Success"
+
+# ---
+
+when isMainModule:
+  setup()
+  update()