You'd need to reimplement LLVM code generation and all. These optimizations only work in that part of the compilation process; do it on your own IR and after lowering to LLVM you'll still want to do it again.
If you're making something that's meant to be load-bearing, something professional that will be used in production, then yeah, just use LLVM. But otherwise I think it's important people feel free to decide the goals and constraints of their hobbies and to pursue their preferences when it comes to their personal projects.
> Let’s say I’m targeting x86 or PTX or some standard subo instruction set. I can do vanilla codegen but it is slow af
and
> Because I want to be self hosted
which struck me as something a hobbyist would say but someone working on a production-grade language and compiler would not.
But you might possibly have found that one weird corner case where it doesn't make sense to depend on a mighty, highly-sophisticated, industrial-strength, extremely well tested and supported, nicely mature, compiler tooling framework.
But that's a pretty big exception to most language implementers who get all of that and more for super-cheap compared to implementing it themselves.
If you're writing a C++ compiler, self-hosting LLVM might not be a massive additional complexity. For any other language it's not an option if self hosting is one of your goals.