iris: Investigate if we can get performance by disabling swizzle when possible
When all shaders are linked together and front/back face colors aren't in use, we could potentially stop using 3DSTATE_SBE::Attribute Swizzle Enable to reorder FS inputs and outputs, and rely on the linker to compact and assign locations for those. It's possible that there's some cost to swizzling. It could also mean re-emitting that packet less, avoiding some CPU and Command Streamer overhead.
But I haven't quantified this idea at all.