Auto merge of #62692 - eddyb:precompute-niches, r=<try>

rustc: precompute the largest Niche and store it in LayoutDetails.

Since we only ever can use at most one niche, it makes sense to just store that in the layout, for the simplest caching (especially as it's almost trivial to compute).

There might be a speedup from this, but even if it's marginal now, the caching would be a more significant benefit for future optimization attempts.

Author: bors