The spec doesn't require implementation to validate indices against
range. This extra validation is costing performance dearly, thus should
be removed.

Bug: angleproject:4373
Change-Id: I61b0628e75de368d9e869008b7c99c429222fd6f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2033495Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>