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/PassManager.h

@@ -107,6 +107,15 @@ private:
     virtual void perform(PassPipelineExecutable&) override;
 };
 
+class Peephole : public Pass {
+public:
+    Peephole() = default;
+    ~Peephole() override = default;
+
+private:
+    virtual void perform(PassPipelineExecutable&) override;
+};
+
 class DumpCFG : public Pass {
 public:
     DumpCFG(FILE* file)