Implement GL_EXT_demote_to_helper_invocation

Test/spv.conditionalDemote.frag

@@ -0,0 +1,18 @@
+#version 460 core
+#extension GL_EXT_demote_to_helper_invocation : enable
+
+layout(set = 0, binding = 0) uniform sampler2D tex;
+layout(location = 0) in vec2 coord;
+layout(location = 0) out vec4 o;
+
+void main (void)
+{
+    vec4 v = texture(tex, coord);
+
+    if (v == vec4(0.1,0.2,0.3,0.4))
+        demote;
+
+    bool x = helperInvocationEXT();
+
+    o = v;
+}