[release-branch.go1.14] reflect: handling flagIndir in DeepEqual potential cycles

For #39607
Fixes #39636

Change-Id: Ia7e597e0da8a193a25382cc633a1c6080b4f7cbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/238361


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
(cherry picked from commit d872bbcf)
Reviewed-on: https://go-review.googlesource.com/c/go/+/238626