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/Label.cpp

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2024, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/BasicBlock.h>
+#include <LibJS/Bytecode/Label.h>
+
+namespace JS::Bytecode {
+
+Label::Label(Bytecode::BasicBlock const& basic_block)
+    : m_address_or_basic_block_index(basic_block.index())
+{
+}
+
+}