LibJS/Bytecode: Store labels as basic block index during compilation

Instead of storing a BasicBlock* and forcing the size of Label to be
sizeof(BasicBlock*), we now store the basic block index as a u32.

This means the final version of the bytecode is able to keep labels
at sizeof(u32), shrinking the size of many instructions. :^)

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -135,7 +135,7 @@ CodeGenerationErrorOr<NonnullGCPtr<Executable>> Generator::generate(VM& vm, ASTN
     }
     for (auto label_offset : label_offsets) {
         auto& label = *reinterpret_cast<Label*>(bytecode.data() + label_offset);
-        auto* block = &label.block();
+        auto* block = generator.m_root_basic_blocks[label.basic_block_index()].ptr();
         label.set_address(block_offsets.get(block).value());
     }