cmd/compile/internal/ir: add DoChildrenWithHidden

cmd/compile/internal/ir: add DoChildrenWithHidden

Analogous to EditChildrenWithHidden.
A commit written by Matthew Dempsky

Old-Change-Id: I9fe0d3ee98d9dbe5f77eb02d666b9f317ee5b6af
Change-Id: I41aacb1545ab3142862b156bd1767fe4a3df4ca0
Reviewed-on: https://go-review.googlesource.com/c/go/+/600497
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>