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 <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go