cmd/compile: remove index out of bounds check in walkIndex

cmd/compile: remove index out of bounds check in walkIndex

Since when any user errors about out-of-bounds constants should have
been already reported by types2.

Change-Id: I81b6bfec53201bbf546c48157fc8154cdb3c6e45
Reviewed-on: https://go-review.googlesource.com/c/go/+/421876
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>