HLSL Legalization: Add scalar replacement

This allow for propagation through structs with dynamically indexed
arrays. This supports recent removal of non-io struct flattening.

SPIRV/GlslangToSpv.cpp

@@ -6086,6 +6086,8 @@ void GlslangToSpv(const glslang::TIntermediate& intermediate, std::vector<unsign
         });
 
         optimizer.RegisterPass(CreateInlineExhaustivePass());
+        optimizer.RegisterPass(CreateEliminateDeadFunctionsPass());
+        optimizer.RegisterPass(CreateScalarReplacementPass());
         optimizer.RegisterPass(CreateLocalAccessChainConvertPass());
         optimizer.RegisterPass(CreateLocalSingleBlockLoadStoreElimPass());
         optimizer.RegisterPass(CreateLocalSingleStoreElimPass());