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.helperInvocation.frag

@@ -0,0 +1,10 @@
+#version 310 es
+precision highp float;
+
+out vec4 outp;
+void main()
+{
+    if (gl_HelperInvocation)
+        ++outp;
+}
+