cmd/compile: fix integer overflow in prove pass

cmd/compile: fix integer overflow in prove pass

The detectSliceLenRelation function incorrectly deduced lower bounds
for "len(s) - i" without checking if the subtraction could overflow
(e.g. when i is negative). This led to incorrect elimination of
bounds checks.

Fixes: #76355
Change-Id: I30ada0e5f1425929ddd8ae1b66e55096ec209b5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/721920
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>