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/CMakeLists.txt

@@ -12,6 +12,7 @@ set(SOURCES
     Bytecode/Pass/DumpCFG.cpp
     Bytecode/Pass/GenerateCFG.cpp
     Bytecode/Pass/MergeBlocks.cpp
+    Bytecode/Pass/Peephole.cpp
     Bytecode/Pass/PlaceBlocks.cpp
     Bytecode/Pass/UnifySameBlocks.cpp
     Bytecode/RegexTable.cpp