GLSL ES 3.0 interface blocks always have uniform storage qualification
and thus can't be used as l-value.

Change-Id: Iffe948dccbf2edb28eb1abe52a6f28dc76a1e84b
Reviewed-on: https://swiftshader-review.googlesource.com/13929Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>