Test for spv1.6 + memory model HelperInvocation

This should generate a Volatile tag on the access, not on the variable
itself.

Test/spv.1.6.helperInvocation.memmodel.frag

@@ -0,0 +1,16 @@
+#version 310 es
+
+#pragma use_vulkan_memory_model
+
+#extension GL_EXT_demote_to_helper_invocation : require
+
+precision highp float;
+
+layout (set=0, binding=0) buffer B {
+   float o;
+};
+
+void main() {
+   demote;
+   o = gl_HelperInvocation ? 1.0 : 0.0;
+}