go/ast: fix bug handling the result of yield in Preorder
Once yield returns false, ast.Preorder must not call yield on any more
nodes. Even after the function passed to ast.Inspect returns false, it
may be invoked again with a non-nil node. Therefore, we must explicitly
truncate the inspection.
For #66339
Change-Id: I2b01e4e96a2d7aca785467c15ab59da13208c161
Reviewed-on: https://go-review.googlesource.com/c/go/+/585520 Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>