JSSpecCompiler: Add if branch merging pass

It merges standalone IfBranch/ElseIfBranch nodes into IfElseIfChain
nodes. This will ease CFG generation later.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Compiler/IfBranchMergingPass.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Compiler/GenericASTPass.h"
+
+namespace JSSpecCompiler {
+
+// IfBranchMergingPass, unsurprisingly, merges if-elseif-else chains, represented as a separate
+// nodes after parsing, into one IfElseIfChain node. It also deals with the following nonsense from
+// the spec:
+// ```
+//   1. If <condition>, then
+//      ...
+//   2. Else,
+//      a. If <condition>, then
+//         ...
+//   3. Else,
+//      ...
+// ```
+class IfBranchMergingPass : public GenericASTPass {
+public:
+    using GenericASTPass::GenericASTPass;
+
+protected:
+    RecursionDecision on_entry(Tree tree) override;
+
+private:
+    static Tree merge_branches(Vector<Tree> const& unmerged_branches);
+};
+
+}