cmd/compile: use memhash from package runtime

cmd/compile: use memhash from package runtime

Simplify the code, and prevent creating ONAME node with nil Func.

Passes toolstash-check.

Change-Id: I5e5be660510dc0ef5521d278c6b9214a80b994eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/436958
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>