glsl: don't set uniform block as used when its not

glsl: don't set uniform block as used when its not

The spec requires unused uniform block to be set as active in the
program resource list. To support this we tell opt dead code not to
remove them. However we can mark them as unused internally and
avoid unnecessarily state changes.

This change is also required for the folowing clean-up patch.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>