Fix constant expression vector indexing.

Change-Id: If3b61ad65f1f1d6687890bd8715b2a534c78a353
Reviewed-on: https://swiftshader-review.googlesource.com/5081
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>