cmd/compile: learn transitive proofs for safe unsigned adds
I've split this into it's own CL to make git bisect more effective.
Change-Id: Iaab5f0bd2ad51e86ced8c6b8fbd371eb75eeef14
Reviewed-on: https://go-review.googlesource.com/c/go/+/685815 Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Mark Freeman <mark@golang.org>