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/baseResults/spv.1.6.helperInvocation.frag.out

@@ -0,0 +1,41 @@
+spv.1.6.helperInvocation.frag
+// Module Version 10600
+// Generated by (magic number): 8000a
+// Id's are bound by 20
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Fragment 4  "main" 8 15
+                              ExecutionMode 4 OriginLowerLeft
+                              Source ESSL 310
+                              Name 4  "main"
+                              Name 8  "gl_HelperInvocation"
+                              Name 15  "outp"
+                              Decorate 8(gl_HelperInvocation) BuiltIn HelperInvocation
+                              Decorate 8(gl_HelperInvocation) Volatile
+                              Decorate 15(outp) Location 0
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeBool
+               7:             TypePointer Input 6(bool)
+8(gl_HelperInvocation):      7(ptr) Variable Input
+              12:             TypeFloat 32
+              13:             TypeVector 12(float) 4
+              14:             TypePointer Output 13(fvec4)
+        15(outp):     14(ptr) Variable Output
+              17:   12(float) Constant 1065353216
+         4(main):           2 Function None 3
+               5:             Label
+               9:     6(bool) Load 8(gl_HelperInvocation)
+                              SelectionMerge 11 None
+                              BranchConditional 9 10 11
+              10:               Label
+              16:   13(fvec4)   Load 15(outp)
+              18:   13(fvec4)   CompositeConstruct 17 17 17 17
+              19:   13(fvec4)   FAdd 16 18
+                                Store 15(outp) 19
+                                Branch 11
+              11:             Label
+                              Return
+                              FunctionEnd