[llvm] [LTO] Avoid assert fail on failed pass plugin load (PR #96691)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

> I'm not 100% sure we shouldn't stop the whole pipeline when the module fails to load. The difference in behavior between opt and llvm-lto2 is probably not a good thing :-/

I'm happy to change it to be like opt.  After all, stopping on failure to load a plugin is what lto already does without this patch (but it's a crash).

https://github.com/llvm/llvm-project/pull/96691