Vulkan always has separate alpha blend.

No test change expected

Bug: b/132280877
Change-Id: Ie76094fcc82070c20b78b846e4d2e30810339f98
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31349Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>