Regres: use Subzero backend by default

Use the Subzero backend instead of LLVM when running Regres per CL.
Turnaround times should be faster, and since Chromium now uses
SwiftShaderVK/Subzero, we'll know which CL breaks it, if any.

Change-Id: If164d87c4946a1f7899c3407cc31fa2f1eba4931
Bug: b/152216043
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44628
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
diff --git a/tests/regres/cmd/regres/main.go b/tests/regres/cmd/regres/main.go
index bfead7c..6a7b2e5 100644
--- a/tests/regres/cmd/regres/main.go
+++ b/tests/regres/cmd/regres/main.go
@@ -1015,7 +1015,7 @@
 		checkoutDir:    checkoutDir,
 		resDir:         resDir,
 		buildDir:       filepath.Join(checkoutDir, "build"),
-		reactorBackend: backendLLVM,
+		reactorBackend: backendSubzero,
 	}
 }