Don't forward uses of an OpIsHelperInvocationEXT op.

If this is computed *before* a `demote`, but used *after*, forwarding it
will produce the wrong value. This does make for uglier shaders, but
it's necessary right now to ensure correctness.

I needed to use an assembly shader to produce the test for this.
`spirv-opt` is not smart enough (or too smart?) to eliminate the
variable that would be used in GLSL to express this.

reference/opt/shaders-msl/vulkan/frag/demote-to-helper.vk.nocompat.msl21.invalid.frag

@@ -5,5 +5,6 @@ using namespace metal;
 
 fragment void main0()
 {
+    bool _9 = simd_is_helper_thread();
 }
 

reference/opt/shaders/vulkan/frag/demote-to-helper-forwarding.asm.vk.nocompat.frag.vk

@@ -0,0 +1,15 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+layout(location = 0) out vec4 FragColor;
+
+void main()
+{
+    bool _15 = helperInvocationEXT();
+    demote;
+    if (!_15)
+    {
+        FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+    }
+}
+

reference/opt/shaders/vulkan/frag/demote-to-helper.vk.nocompat.frag.vk

@@ -4,6 +4,7 @@
 void main()
 {
     demote;
-    bool helper = helperInvocationEXT();
+    bool _9 = helperInvocationEXT();
+    bool helper = _9;
 }
 

reference/shaders-msl/vulkan/frag/demote-to-helper.vk.nocompat.msl21.invalid.frag

@@ -5,6 +5,7 @@ using namespace metal;
 
 fragment void main0()
 {
-    bool helper = simd_is_helper_thread();
+    bool _9 = simd_is_helper_thread();
+    bool helper = _9;
 }
 

reference/shaders/vulkan/frag/demote-to-helper-forwarding.asm.vk.nocompat.frag.vk

@@ -0,0 +1,15 @@
+#version 450
+#extension GL_EXT_demote_to_helper_invocation : require
+
+layout(location = 0) out vec4 FragColor;
+
+void main()
+{
+    bool _15 = helperInvocationEXT();
+    demote;
+    if (!_15)
+    {
+        FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+    }
+}
+

reference/shaders/vulkan/frag/demote-to-helper.vk.nocompat.frag.vk

@@ -4,6 +4,7 @@
 void main()
 {
     demote;
-    bool helper = helperInvocationEXT();
+    bool _9 = helperInvocationEXT();
+    bool helper = _9;
 }
 

shaders/vulkan/frag/demote-to-helper-forwarding.asm.vk.nocompat.frag

@@ -0,0 +1,41 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Khronos Glslang Reference Front End; 7
+; Bound: 19
+; Schema: 0
+               OpCapability Shader
+               OpCapability DemoteToHelperInvocationEXT
+               OpExtension "SPV_EXT_demote_to_helper_invocation"
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %main "main" %FragColor
+               OpExecutionMode %main OriginUpperLeft
+               OpSource GLSL 450
+               OpSourceExtension "GL_EXT_demote_to_helper_invocation"
+               OpName %main "main"
+               OpName %FragColor "FragColor"
+               OpDecorate %FragColor Location 0
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+       %bool = OpTypeBool
+%_ptr_Function_bool = OpTypePointer Function %bool
+      %float = OpTypeFloat 32
+    %v4float = OpTypeVector %float 4
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+  %FragColor = OpVariable %_ptr_Output_v4float Output
+    %float_1 = OpConstant %float 1
+    %float_0 = OpConstant %float 0
+         %19 = OpConstantComposite %v4float %float_1 %float_0 %float_0 %float_1
+       %main = OpFunction %void None %3
+          %5 = OpLabel
+          %9 = OpIsHelperInvocationEXT %bool
+               OpDemoteToHelperInvocationEXT
+         %10 = OpLogicalNot %bool %9
+               OpSelectionMerge %12 None
+               OpBranchConditional %10 %11 %12
+         %11 = OpLabel
+               OpStore %FragColor %19
+               OpBranch %12
+         %12 = OpLabel
+               OpReturn
+               OpFunctionEnd

spirv_glsl.cpp

@@ -9789,7 +9789,7 @@ void CompilerGLSL::emit_instruction(const Instruction &instruction)
 		if (!options.vulkan_semantics)
 			SPIRV_CROSS_THROW("GL_EXT_demote_to_helper_invocation is only supported in Vulkan GLSL.");
 		require_extension_internal("GL_EXT_demote_to_helper_invocation");
-		emit_op(ops[0], ops[1], "helperInvocationEXT()", true);
+		emit_op(ops[0], ops[1], "helperInvocationEXT()", false);
 		break;
 
 	default:

spirv_msl.cpp

@@ -4473,7 +4473,7 @@ void CompilerMSL::emit_instruction(const Instruction &instruction)
 			SPIRV_CROSS_THROW("simd_is_helper_thread() is only supported on macOS.");
 		else if (msl_options.is_macos() && !msl_options.supports_msl_version(2, 1))
 			SPIRV_CROSS_THROW("simd_is_helper_thread() requires version 2.1 on macOS.");
-		emit_op(ops[0], ops[1], "simd_is_helper_thread()", true);
+		emit_op(ops[0], ops[1], "simd_is_helper_thread()", false);
 		break;
 
 	default: