cmd/compile: teach prove about len's & cap's max based on the element size

cmd/compile: teach prove about len's & cap's max based on the element size

Change-Id: I88056fada1ff488c199fce54cf737dbdd091214d
Reviewed-on: https://go-review.googlesource.com/c/go/+/695095
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>