cmd/link: record only the first occurance in Reachparent graph

cmd/link: record only the first occurance in Reachparent graph

In the deadcode pass, a type symbol may be marked twice, one
without UsedInIface, one with. For the second time, don't
update the Reachparent graph, so it only records the path of
the first time the symbol is reached. This ensures the
Reachparent graph is acyclic.

TODO: add a test. (This only affects GOEXPERIMENT=fieldtrack)

Change-Id: I68e8a1a69c3830bc8aee5df946151dc22dcb2b29
Reviewed-on: https://go-review.googlesource.com/c/go/+/254297
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>