[clang] [lld] [lld][LTO] Respect `-fno-slp-vectorize` for the LTO pipeline (PR #201585)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I had that thought, but the current control is pretty much this, which means that adding a module flag would require some more invasive changes to how the pass functions because this level of config doesn't touch IR at all. I can try to add a function level attribute or similar for it and make the pass exit early if you think that's cleaner.
> 

This is a *much* larger change that should not be done in isolation for this one flag, and would need a fleshed out design and RFC. I do not think pass configuration belongs in the program 

https://github.com/llvm/llvm-project/pull/201585