Fix broken optimization of alpha VK_BLEND_OP_REVERSE_SUBTRACT

Test: dEQP-VK.pipeline.blend.*
Bug: b/118386749
Change-Id: I382b6101980171fec6b7fad4d2a126b702526f8b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30551
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Device/Context.cpp

diff --git a/src/Device/Context.cpp b/src/Device/Context.cpp
index 8d460c8..ce4fc3e 100644
--- a/src/Device/Context.cpp
+++ b/src/Device/Context.cpp
@@ -590,7 +590,7 @@
 					}
 					else
 					{
-						return VK_BLEND_OP_ZERO_EXT;
+						return VK_BLEND_OP_REVERSE_SUBTRACT;
 					}
 				}
 			case VK_BLEND_OP_MIN: