Optimize canonicalizer flag checks. The most important change here relates to type folding: we now check the flags up front, instead of doing it in `inner_fold_ty` after checking the cache and doing a match. This is a small perf win, and matches other similar folders (e.g. `CanonicalInstantiator`). Likewise for const folding, we now check the flags first. (There is no cache for const folding.) Elsewhere we don't check flags before folding a predicate (unnecessary, because `fold_predicate` already checks the flags itself before doing anything else), and invert the flag checks in a couple of methods to match the standard order. r? @lcnr