Fix #12785 (#12943)

* Fix #12785 and add test

* better variable name

tests/casestmt/t12785.nim

@@ -0,0 +1,47 @@
+discard """
+  cmd: '''nim c --newruntime $file'''
+  output: '''copied
+copied
+2
+copied
+copied
+2
+destroyed
+destroyed'''
+"""
+
+type
+  ObjWithDestructor = object
+    a: int
+proc `=destroy`(self: var ObjWithDestructor) =
+  echo "destroyed"
+
+proc `=`(self: var ObjWithDestructor, other: ObjWithDestructor) =
+  echo "copied"
+
+proc test(a: range[0..1], arg: ObjWithDestructor) =
+  var iteration = 0
+  while true:
+    {.computedGoto.}
+
+    let
+      b = int(a) * 2
+      c = a
+      d = arg
+      e = arg
+
+    discard c
+    discard d
+    discard e
+
+    inc iteration
+
+    case a
+    of 0:  
+        assert false
+    of 1:      
+      echo b
+      if iteration == 2:
+        break
+
+test(1, ObjWithDestructor())