Rollup merge of #69022 - ljedrz:traits_tweak_vecs, r=petrochenkov

traits: preallocate 2 Vecs of known initial size

The 2 preallocations are pretty obvious; both vectors will be as big as or larger than the collections they are created from.

In `WfPredicates::normalize` the change from a functional style improves readability and should be perf-friendly, too.

diff --cc src/librustc/traits/select.rs
Simple merge