cmd/compile: propagate unsigned limits for Div and Mod if arguments are positive
I didn't implemented negative limits since prove is most useful for BCE which
should never be negative in the first place.
Change-Id: I302ee462cdc20bd4edff0618f7e49ff66fc2a007
Reviewed-on: https://go-review.googlesource.com/c/go/+/605136
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>