cmd/compile: fix mis-infer bounds in slice len/cap calculations

cmd/compile: fix mis-infer bounds in slice len/cap calculations

CL 704875 enhanced prove to infer bounds when index have a relationship
with len(A) - K. However, the change incorrectly infer "K - len(A)" case,
causing wrong bounds information.

Fixing this by matching exactly "len(A) - K" case.

Fixes #76709

Change-Id: Ibeedff55520658401af5bd3aa7e98cc1bcf38fd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/727180
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jakub Ciolek <jakub@ciolek.dev>
Reviewed-by: David Chase <drchase@google.com>