Precompute into select arms (#6212)

E.g.

(i32.add
  (select
    (i32.const 100)
    (i32.const 200)
    (..condition..)
  )
  (i32.const 50)
)

;; =>

(select
  (i32.const 150)
  (i32.const 250)
  (..condition..)
)

We cannot fully precompute the select, but we can "partially precompute" it, by precomputing
its arms using the parent.

This may require looking several steps up the parent chain, which is an awkward operation in
our simple walkers, so to do it we capture stacks of parents and operate directly on them. This
is a little slower than a normal walk, so only do it when we see a promising select, and only in
-O2 and above (this makes the pass 7% or so slower; not a large cost, but best to avoid it in
-O1).

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