If a SSBO is active but not statically used, there will be no UAV slot is
bound. So we should skip this kind of block.

Bug: angleproject:1951
Change-Id: I4d813ddefcce6c31fa02701f26148eb21c00f380
Reviewed-on: https://chromium-review.googlesource.com/c/1381847Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>