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/SpvBuilder.cpp

@@ -2130,6 +2130,35 @@ Id Builder::accessChainGetInferredType()
     return type;
 }
 
+// comment in header
+void Builder::eliminateDeadDecorations() {
+    std::unordered_set<const Block*> reachable_blocks;
+    std::unordered_set<Id> unreachable_definitions;
+    // Collect IDs defined in unreachable blocks. For each function, label the
+    // reachable blocks first. Then for each unreachable block, collect the
+    // result IDs of the instructions in it.
+    for (auto& f : module.getFunctions()) {
+        Block* entry = f->getEntryBlock();
+        inReadableOrder(entry, [&reachable_blocks](const Block* b) {
+            reachable_blocks.insert(b);
+        });
+        for (auto& b : f->getBlocks()) {
+            if (!reachable_blocks.count(b)) {
+                for (auto& i : b->getInstructions()) {
+                    unreachable_definitions.insert(i->getResultId());
+                }
+            }
+        }
+    }
+    decorations.erase(std::remove_if(decorations.begin(), decorations.end(),
+        [&unreachable_definitions](std::unique_ptr<Instruction>& I) {
+            Instruction* inst = I.get();
+            Id decoration_id = inst->getIdOperand(0);
+            return unreachable_definitions.count(decoration_id) != 0;
+        }),
+        decorations.end());
+}
+
 void Builder::dump(std::vector<unsigned int>& out) const
 {
     // Header, before first instructions: