LibJS: Add a basic pass manager and add some basic passes

This commit adds a bunch of passes, the most interesting of which is a
pass that merges blocks together, and a pass that places blocks that
flow into each other next to each other, and a very simply pass that
removes duplicate basic blocks.
Note that this does not remove the jump at the end of each block in that
pass to avoid scope creep in the passes.

Userland/Libraries/LibJS/Bytecode/Pass/PlaceBlocks.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/PassManager.h>
+
+namespace JS::Bytecode::Passes {
+
+void PlaceBlocks::perform(PassPipelineExecutable& executable)
+{
+    started();
+
+    VERIFY(executable.cfg.has_value());
+    auto cfg = executable.cfg.release_value();
+
+    Vector<BasicBlock&> replaced_blocks;
+    HashTable<BasicBlock const*> reachable_blocks;
+
+    // Visit the blocks in CFG order
+    AK::Function<void(BasicBlock const*)> visit = [&](auto* block) {
+        if (reachable_blocks.contains(block))
+            return;
+
+        reachable_blocks.set(block);
+        replaced_blocks.append(*const_cast<BasicBlock*>(block));
+
+        for (auto& entry : cfg.get(block).value_or({}))
+            visit(entry);
+    };
+
+    // Make sure to visit the entry block first
+    visit(&executable.executable.basic_blocks.first());
+
+    for (auto& entry : cfg)
+        visit(entry.key);
+
+    // Put the unreferenced blocks back in at the end
+    for (auto& entry : static_cast<Vector<NonnullOwnPtr<BasicBlock>>&>(executable.executable.basic_blocks)) {
+        if (reachable_blocks.contains(entry.ptr()))
+            (void)entry.leak_ptr();
+        else
+            replaced_blocks.append(*entry.leak_ptr()); // Don't try to do DCE here.
+    }
+
+    executable.executable.basic_blocks.clear();
+    for (auto& block : replaced_blocks)
+        executable.executable.basic_blocks.append(adopt_own(block));
+
+    finished();
+}
+
+}