Rollup merge of #144390 - oli-obk:arbitrary-enum-discrs, r=SparrowLii

Remove dead code and extend test coverage and diagnostics around it

I was staring a bit at the `dont_niche_optimize_enum` variable and figured out that part of it is dead code (at least today it is). I changed the diagnostic and test around the code that makes that part dead code, so everything that makes removing that code sound is visible in this PR

Author: matthiaskrgr