Use this to disable reads and writes on compute shader lanes that are not part of the subgroup.

Bug: b/126871859
Bug: b/128527271
Change-Id: Idd7ad240a8f09e6e47db34b6ed5b0ec7ba959d39
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27009
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>