Fix transformation of yield in inline context (#9135)

When the loop variables are part of the envP block it is not safe to use
a nkFastAsgn.

Fixes #2656

tests/cnstseq/t2656.nim

@@ -0,0 +1,35 @@
+discard """
+  output: '''
+onetwothree
+onetwothree
+onetwothree
+one1two2three3
+'''
+"""
+
+iterator it1(args: seq[string]): string =
+  for s in args: yield s
+iterator it2(args: seq[string]): string {.closure.} =
+  for s in args: yield s
+iterator it3(args: openArray[string]): string {.closure.} =
+  for s in args: yield s
+iterator it4(args: openArray[(string, string)]): string {.closure.} =
+  for s1, s2 in items(args): yield s1 & s2
+
+block:
+  const myConstSeq = @["one", "two", "three"]
+  for s in it1(myConstSeq):
+    stdout.write s
+  echo ""
+  for s in it2(myConstSeq):
+    stdout.write s
+  echo ""
+  for s in it3(myConstSeq):
+    stdout.write s
+  echo ""
+
+block:
+  const myConstSeq = @[("one", "1"), ("two", "2"), ("three", "3")]
+  for s in it4(myConstSeq):
+    stdout.write s
+  echo ""