LibJS/Bytecode: Bring back the bytecode optimization pipeline

...minus the EliminateLoads pass, since it was not compatible with the
new bytecode format.

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

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/PassManager.h>
+#include <string.h>
+
+namespace JS::Bytecode::Passes {
+
+void UnifySameBlocks::perform(PassPipelineExecutable& executable)
+{
+    started();
+
+    VERIFY(executable.cfg.has_value());
+    VERIFY(executable.inverted_cfg.has_value());
+    auto cfg = executable.cfg.release_value();
+    auto inverted_cfg = executable.inverted_cfg.release_value();
+
+    HashMap<BasicBlock const*, BasicBlock const*> equal_blocks;
+
+    for (size_t i = 0; i < executable.executable.basic_blocks.size(); ++i) {
+        auto& block = executable.executable.basic_blocks[i];
+        auto block_bytes = block->instruction_stream();
+        for (auto& candidate_block : executable.executable.basic_blocks.span().slice(i + 1)) {
+            if (equal_blocks.contains(&*candidate_block))
+                continue;
+            // FIXME: This can probably be relaxed a bit...
+            if (candidate_block->size() != block->size())
+                continue;
+            if (candidate_block->finalizer() != block->finalizer())
+                continue;
+            if (candidate_block->handler() != block->handler())
+                continue;
+
+            auto candidate_bytes = candidate_block->instruction_stream();
+            if (memcmp(candidate_bytes.data(), block_bytes.data(), candidate_block->size()) == 0)
+                equal_blocks.set(candidate_block.ptr(), block);
+        }
+    }
+
+    auto replace_blocks = [&](auto& match, auto& replacement) {
+        Optional<size_t> first_successor_position;
+        auto it = executable.executable.basic_blocks.find_if([match](auto& block) { return match == block; });
+        VERIFY(!it.is_end());
+        executable.executable.basic_blocks.remove(it.index());
+        if (!first_successor_position.has_value())
+            first_successor_position = it.index();
+
+        for (auto& block : executable.executable.basic_blocks) {
+            InstructionStreamIterator it { block->instruction_stream() };
+            while (!it.at_end()) {
+                auto& instruction = *it;
+                ++it;
+                const_cast<Instruction&>(instruction).replace_references(*match, replacement);
+            }
+        }
+        return first_successor_position;
+    };
+
+    for (auto& entry : equal_blocks)
+        (void)replace_blocks(entry.key, *entry.value);
+
+    finished();
+}
+
+}