Performance regression with niche optimization

[mikebenfield]

After PR #94075 for more niche optimizations was merged, there were some performance regressions.

Some performance regressions are possibly just due to the extra arithmetic and branch or cmov required when getting a discriminant out of a tag. But notably when compiling syn, the regression is largely due to extra time in LLVM_lto_optimize.

It would be nice to understand this better, and ideally do something about it.

[Rageking8]

@rustbot label +T-compiler +regression-untriaged

[nnethercote]

#102035 is a related example, where increased use of niche-filling causes some small but widespread instruction count regressions.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium