Fix allowing fragment shader invariant declarations.

Bug 18732025

Change-Id: I8a3024327533c3e6db72248caafc5e618808edf5
Reviewed-on: https://swiftshader-review.googlesource.com/1630
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>