Remove Emit entry in favor of auto-emitting code

All instructions but OpVariable and OpLabel are automatically emitted.
These functions have to call AddLocalVariable/AddGlobalVariable or
AddLabel respectively.

src/instructions/flow.cpp

@@ -30,7 +30,7 @@ Id Module::OpSelectionMerge(Id merge_block, spv::SelectionControlMask selection_
 }
 
 Id Module::OpLabel() {
-    return AddCode(spv::Op::OpLabel, bound++);
+    return code_store.emplace_back(std::make_unique<Op>(spv::Op::OpLabel, bound++)).get();
 }
 
 Id Module::OpBranch(Id target_label) {

src/instructions/memory.cpp

@@ -16,7 +16,7 @@ Id Module::OpVariable(Id result_type, spv::StorageClass storage_class, Id initia
     if (initializer) {
         op->Add(initializer);
     }
-    return AddCode(std::move(op));
+    return code_store.emplace_back(std::move(op)).get();
 }
 
 Id Module::OpLoad(Id result_type, Id pointer, std::optional<spv::MemoryAccessMask> memory_access) {

src/sirit.cpp

@@ -98,22 +98,24 @@ void Module::AddExecutionMode(Id entry_point, spv::ExecutionMode mode,
     execution_modes.push_back(std::move(op));
 }
 
-Id Module::Emit(Id op) {
-    assert(op != nullptr);
-    code.push_back(op);
-    return op;
+Id Module::AddLabel(Id label) {
+    assert(label != nullptr);
+    return code.emplace_back(label);
+}
+
+Id Module::AddLocalVariable(Id variable) {
+    assert(variable != nullptr);
+    return code.emplace_back(variable);
 }
 
 Id Module::AddGlobalVariable(Id variable) {
     assert(variable);
-    global_variables.push_back(variable);
-    return variable;
+    return global_variables.emplace_back(variable);
 }
 
 Id Module::AddCode(std::unique_ptr<Op> op) {
-    const auto id = op.get();
-    code_store.push_back(std::move(op));
-    return id;
+    const Id id = code_store.emplace_back(std::move(op)).get();
+    return code.emplace_back(id);
 }
 
 Id Module::AddCode(spv::Op opcode, std::optional<u32> id) {