cmd/compile: learn transitive proofs for safe positive signed adds
I've split this into it's own CL to make git bisect more effective.
Change-Id: I3fbb42ec7d29169a29f7f55ef2c188317512f532
Reviewed-on: https://go-review.googlesource.com/c/go/+/685819
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>