The shader is run multiple times reusing the same routine state.
We need to set the mask correctly before each execution.

Test: dEQP-VK.glsl.discard.*
Change-Id: Ic34385715cc4a16d931a3ff4fae4eeaf48d52223
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30328Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>