The implementation of ballotARB() will start by zeroing the flags
register. So, a doing something like

        if (gl_SubGroupInvocationARB % 2u == 0u) {
                ... = ballotARB(true);
		[...]
        } else {
                ... = ballotARB(true);
		[...]
	}

(like fs-ballot-if-else.shader_test does) would generate identical MOVs
to the same destination (the flag register!), and we definitely do not
want to pull that out of the control flow.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>