Add test for indexing on variables whose types are constrained to
various kinds of types.
Change-Id: I991eecfe39dba5d817c0fbe259ba558d4881ea84
Reviewed-on: https://go-review.googlesource.com/c/go/+/360867
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
