dEQP doesn't try to render this format with blending enabled, so we
missed the attachment read path.

Change-Id: Ib1decc0dbdf1e9895b5ac00b478a65fd717e534c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36488Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>