Add support for spirv1.6

Add command line support which enables the following behavior:

- Remap discard

  Map discard to DemoteToHelperInvocation for HLSL shaders. Map to
  OpTerminateInvocation for GLSL shaders.

- Decorate HelperInvocation with Volatile

- Use localSizeId for execution mode

  WorkGroupSize is deprecated in spirv1.6

Also update known goods to SPIRV 1.6

Test/spv.1.6.conditionalDiscard.frag

@@ -0,0 +1,14 @@
+#version 400
+
+uniform sampler2D tex;
+in vec2 coord;
+
+void main (void)
+{
+    vec4 v = texture(tex, coord);
+
+    if (v == vec4(0.1,0.2,0.3,0.4))
+        discard;
+
+    gl_FragColor = v;
+}