LibJS/JIT: Support the NewString bytecode op

This necessitated making the JIT::Compiler aware of the current
Bytecode::Executable, since that's where all the string literals are
held, but that seems like a good thing.

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -157,6 +157,8 @@ public:
     ThrowCompletionOr<void> execute_impl(Bytecode::Interpreter&) const;
     DeprecatedString to_deprecated_string_impl(Bytecode::Executable const&) const;
 
+    StringTableIndex index() const { return m_string; }
+
 private:
     StringTableIndex m_string;
 };