change `wf.rs` to use a `TypeVisitor` instead of `walk`

[lcnr]

we should change the following function to use a TypeVisitor instead of walk

rust/compiler/rustc_trait_selection/src/traits/wf.rs

Line 512 in b6e4299

fn compute(&mut self, arg: GenericArg<'tcx>) {

Using a walker causes the control-flow to be a lot more complicated and given that this code is soundness-critical, we should make sure it's as easy to understand as possible

[ShoyuVanilla]

@rustbot claim