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.cpp

@@ -0,0 +1,97 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/TypeCasts.h>
+
+#include "AST/AST.h"
+#include "Compiler/IfBranchMergingPass.h"
+
+namespace JSSpecCompiler {
+
+RecursionDecision IfBranchMergingPass::on_entry(Tree tree)
+{
+    if (auto list = as<TreeList>(tree); list) {
+        Vector<Tree> result;
+        Vector<Tree> unmerged_branches;
+
+        auto merge_if_needed = [&] {
+            if (!unmerged_branches.is_empty()) {
+                result.append(merge_branches(unmerged_branches));
+                unmerged_branches.clear();
+            }
+        };
+
+        for (auto const& node : list->m_expressions) {
+            if (is<IfBranch>(node.ptr())) {
+                merge_if_needed();
+                unmerged_branches.append(node);
+            } else if (is<ElseIfBranch>(node.ptr())) {
+                unmerged_branches.append(node);
+            } else {
+                merge_if_needed();
+                result.append(node);
+            }
+        }
+        merge_if_needed();
+
+        list->m_expressions = move(result);
+    }
+    return RecursionDecision::Recurse;
+}
+
+Tree IfBranchMergingPass::merge_branches(Vector<Tree> const& unmerged_branches)
+{
+    static const Tree error = make_ref_counted<ErrorNode>("Cannot make sense of if-elseif-else chain"sv);
+
+    VERIFY(unmerged_branches.size() >= 1);
+
+    Vector<Tree> conditions;
+    Vector<Tree> branches;
+    NullableTree else_branch;
+
+    if (auto if_branch = as<IfBranch>(unmerged_branches[0]); if_branch) {
+        conditions.append(if_branch->m_condition);
+        branches.append(if_branch->m_branch);
+    } else {
+        return error;
+    }
+
+    for (size_t i = 1; i < unmerged_branches.size(); ++i) {
+        auto branch = as<ElseIfBranch>(unmerged_branches[i]);
+
+        if (!branch)
+            return error;
+
+        if (!branch->m_condition) {
+            // There might be situation like:
+            //   1. If <condition>, then
+            //      ...
+            //   2. Else,
+            //      a. If <condition>, then
+            //         ...
+            //   3. Else,
+            //      ...
+            auto substep_list = as<TreeList>(branch->m_branch);
+            if (substep_list && substep_list->m_expressions.size() == 1) {
+                if (auto nested_if = as<IfBranch>(substep_list->m_expressions[0]); nested_if)
+                    branch = make_ref_counted<ElseIfBranch>(nested_if->m_condition, nested_if->m_branch);
+            }
+        }
+
+        if (branch->m_condition) {
+            conditions.append(branch->m_condition.release_nonnull());
+            branches.append(branch->m_branch);
+        } else {
+            if (i + 1 != unmerged_branches.size())
+                return error;
+            else_branch = branch->m_branch;
+        }
+    }
+
+    return make_ref_counted<IfElseIfChain>(move(conditions), move(branches), else_branch);
+}
+
+}