SPV: Change barrier emission to conform to Khronos decisions.

The memory model group agreed to these definitions for how
to map GLSL barrier, memoryBarrier, etc. With HLSL following suit.

Test/spv.barrier.vert

@@ -0,0 +1,15 @@
+#version 450
+
+layout(location=0) out vec4 c0;
+layout(location=1) out vec4 c1;
+
+void main()
+{
+    c0 = vec4(1.0);
+    memoryBarrier();
+    c1 = vec4(1.0);
+    memoryBarrierBuffer();
+    ++c0;
+    memoryBarrierImage();
+    ++c0;
+}