If robustness is disabled, don't bother zeroing disabled lanes.
Gives the LLVM optimizer & backend some more wiggle room.

Bug: b/135609394
Change-Id: I425eba2b48601a35dc40aa8ca071e6e61f35024f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33708
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>