Fix ComputeDispatchTests

BUG=dawn:710

Change-Id: I47dac524f66f84e2d5e3aa966935bd2ca9d03144
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/44920
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/tests/end2end/ComputeDispatchTests.cpp

@@ -45,7 +45,7 @@ class ComputeDispatchTests : public DawnTest {
             fn main() -> void {
                 const dispatch : vec3<u32> = input.expectedDispatch;
 
-                if (dispatch.x == 0 || dispatch.y == 0 || dispatch.z == 0) {
+                if (dispatch.x == 0u || dispatch.y == 0u || dispatch.z == 0u) {
                     output.workGroups = vec3<u32>(0xFFFFFFFFu, 0xFFFFFFFFu, 0xFFFFFFFFu);
                     return;
                 }