Remove assert to accept OpAccessChain without indexes.

Change-Id: Ie57f60d6ac35198e7d7a1b75071fcd985dc1da18
Bug: b/161516333
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54728Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>