Let's just initialize these to zero, and let the optimizer remove redundant assignments.

Bug: b/136149446
Change-Id: Ia776bae4433f78f5a66b16f275e728a70dd4e18d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33468Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>