HLSL: Does not support case-fallthrough.

Disable any fallthrough on HLSL. Risky business if fallthrough blocks
had a barrier(), but can't do anything about that ...

reference/shaders-hlsl-no-opt/asm/frag/switch-block-case-fallthrough.asm.frag

@@ -30,16 +30,23 @@ void frag_main()
         case 0:
         {
             _30 = 3;
+            j = _30;
+            _31 = 0;
+            j = _31;
+            break;
         }
         default:
         {
             j = _30;
             _31 = 0;
+            j = _31;
+            break;
         }
         case 1:
         case 11:
         {
             j = _31;
+            break;
         }
         case 2:
         {
@@ -59,6 +66,8 @@ void frag_main()
         }
         case 4:
         {
+            i = 0;
+            break;
         }
         case 5:
         {

spirv_glsl.cpp

@@ -11702,7 +11702,7 @@ void CompilerGLSL::emit_block_chain(SPIRBlock &block)
 			}
 
 			auto &case_block = get<SPIRBlock>(target_block);
-			if (i + 1 < num_blocks &&
+			if (backend.support_case_fallthrough && i + 1 < num_blocks &&
 			    execution_is_direct_branch(case_block, get<SPIRBlock>(block_declaration_order[i + 1])))
 			{
 				// We will fall through here, so just terminate the block chain early.

spirv_glsl.hpp

@@ -405,6 +405,7 @@ protected:
 		bool comparison_image_samples_scalar = false;
 		bool native_pointers = false;
 		bool support_small_type_sampling_result = false;
+		bool support_case_fallthrough = true;
 	} backend;
 
 	void emit_struct(SPIRType &type);

spirv_hlsl.cpp

@@ -4700,6 +4700,7 @@ string CompilerHLSL::compile()
 	backend.can_declare_arrays_inline = false;
 	backend.can_return_array = false;
 	backend.nonuniform_qualifier = "NonUniformResourceIndex";
+	backend.support_case_fallthrough = false;
 
 	fixup_type_alias();
 	reorder_type_alias();