Remove decoration of undefined IDs

Fix issue #185 by removing OpDecorate instructions whose target IDs are
defined in unreachable blocks and thus not dumped in the generated
SPIR-V code.

SPIRV/GlslangToSpv.cpp

@@ -719,6 +719,7 @@ void TGlslangToSpvTraverser::dumpSpv(std::vector<unsigned int>& out)
     for (auto it = iOSet.cbegin(); it != iOSet.cend(); ++it)
         entryPoint->addIdOperand(*it);
 
+    builder.eliminateDeadDecorations();
     builder.dump(out);
 }