LibJS/Bytecode: Add peephole optimization pass and fuse compare+jump

This patch adds a new "Peephole" pass for performing small, local
optimizations to bytecode.

We also introduce the first such optimization, fusing a sequence of
some comparison instruction FooCompare followed by a JumpIf into a
new set of JumpFooCompare instructions.

This gives a ~50% speed-up on the following microbenchmark:

    for (let i = 0; i < 10_000_000; ++i) {
    }

But more traditional benchmarks see a pretty sizable speed-up as well,
for example 15% on Kraken/ai-astar.js and 16% on Kraken/audio-dft.js :^)

Userland/Libraries/LibJS/Bytecode/BasicBlock.h

@@ -82,15 +82,15 @@ public:
         op->set_source_record({ start_offset, end_offset });
     }
 
-private:
-    explicit BasicBlock(String name);
-
     void terminate(size_t slot_offset)
     {
         m_terminated = true;
         m_terminator_offset = slot_offset;
     }
 
+private:
+    explicit BasicBlock(String name);
+
     Vector<u8> m_buffer;
     BasicBlock const* m_handler { nullptr };
     BasicBlock const* m_finalizer { nullptr };