Iterator fixes (#11234)

tests/iter/titer13.nim

@@ -0,0 +1,72 @@
+discard """
+  output: '''b yields
+c yields
+a returns
+b yields
+b returns
+c yields
+
+
+1
+2
+3
+4
+'''
+"""
+
+block:
+  template tloop(iter: untyped) =
+    for i in iter():
+      echo i
+
+  template twhile(iter: untyped) =
+    let it = iter
+    while not finished(it):
+      echo it()
+
+  iterator a(): auto {.closure.} =
+    if true: return "a returns"
+    yield "a yields"
+
+  iterator b(): auto {.closure.} =
+    yield "b yields"
+    if true: return "b returns"
+
+  iterator c(): auto {.closure.} =
+    yield "c yields"
+    if true: return
+
+  iterator d(): auto {.closure.} =
+    if true: return
+    yield "d yields"
+
+  tloop(a)
+  tloop(b)
+  tloop(c)
+  tloop(d)
+  twhile(a)
+  twhile(b)
+  twhile(c)
+  twhile(d)
+
+block:
+  iterator a: auto =
+    yield 1
+  for x in a():
+    echo x
+
+  let b = iterator: int =
+    yield 2
+  for x in b():
+    echo x
+
+  let c = iterator: auto =
+    yield 3
+  for x in c():
+    echo x
+
+block:
+  iterator myIter2(): auto {.closure.} =
+    yield 4
+  for a in myIter2():
+    echo a