dec inLoop after exiting the while scope in computeLiveRanges [backport] (#19918)

* dec inLoop after exiting the while scope in computeLiveRanges

* add testcase

compiler/varpartitions.nim

@@ -851,7 +851,7 @@ proc computeLiveRanges(c: var Partitions; n: PNode) =
     #     connect(graph, cursorVar)
     inc c.inLoop
     for child in n: computeLiveRanges(c, child)
-    inc c.inLoop
+    dec c.inLoop
   of nkElifBranch, nkElifExpr, nkElse, nkOfBranch:
     inc c.inConditional
     for child in n: computeLiveRanges(c, child)

tests/arc/tcursorloop.nim

@@ -0,0 +1,45 @@
+discard """
+  cmd: '''nim c --gc:arc --expandArc:traverse --hint:Performance:off $file'''
+  nimout: '''
+--expandArc: traverse
+
+var
+  it
+  jt_cursor
+try:
+  `=copy`(it, root)
+  block :tmp:
+    while (
+      not (it == nil)):
+      if true:
+        echo [it.s]
+        `=copy`(it, it.ri)
+  jt_cursor = root
+  if (
+    not (jt_cursor == nil)):
+    echo [jt_cursor.s]
+    jt_cursor = jt_cursor.ri
+finally:
+  `=destroy`(it)
+-- end of expandArc ------------------------
+'''
+"""
+
+type
+  Node = ref object
+    le, ri: Node
+    s: string
+
+proc traverse(root: Node) =
+  var it = root
+  while it != nil:
+    if true:
+      echo it.s
+      it = it.ri
+
+  var jt = root
+  if jt != nil:
+    echo jt.s
+    jt = jt.ri
+
+traverse(nil)