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/+/44628Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>