cmd/compile: enhance prove to infer bounds in slice len/cap calculations

cmd/compile: enhance prove to infer bounds in slice len/cap calculations

the example comes up in chunked reslicing, e.g. A[i:] where i
has a relationship with len(A)-K.

Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.

Change-Id: Ib97dede6cfc7bbbd27b4f384988f741760686604
Reviewed-on: https://go-review.googlesource.com/c/go/+/704875
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708863
Reviewed-by: David Chase <drchase@google.com>