cmd/compile: fix bounds check report

cmd/compile: fix bounds check report

For constant-index, variable length situations.

Inadvertant shadowing of the yVal variable. Oops.

Fixes #75327

Change-Id: I3403066fc39b7664222a3098cf0f22b5761ea66a
Reviewed-on: https://go-review.googlesource.com/c/go/+/702015
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>