Fix killMask initialization
Fix killMask initialization

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/+/30328
Tested-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>
Presubmit-Ready: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader