fixes #19457 seqs are not properly updated in loop with ARC/ORC (#19922)

* test CI

* fixes #19457

* add comments

Co-authored-by: sandytypical <43030857+xflywind@users.noreply.github.com>

tests/arc/t19457.nim

@@ -0,0 +1,16 @@
+discard """
+  matrix: "--gc:refc; --gc:arc"
+"""
+
+# bug #19457
+proc gcd(x, y: seq[int]): seq[int] =
+    var
+      a = x
+      b = y
+    while b[0] > 0:
+      let c = @[a[0] mod b[0]]
+      a = b
+      b = c
+    return a
+
+doAssert gcd(@[1], @[2]) == @[1]