Improve selectify

I think I already mentioned a similar problem, but it was a long time ago.

If we have an "if" branch without a false arm, you selectify stops working:

function selectify_early_return(x: i32): i32 {
  if (x) return x;
  return 1;
}

Just add this test into lit/remove-unused-brs.wast as:

  ;; CHECK:      (func $selectify-early-return (param $x i32) (result i32)
  ;; CHECK-NEXT:  (if
  ;; CHECK-NEXT:   (local.get $x)
  ;; CHECK-NEXT:   (return
  ;; CHECK-NEXT:    (local.get $x)
  ;; CHECK-NEXT:   )
  ;; CHECK-NEXT:  )
  ;; CHECK-NEXT:  (i32.const 1)
  ;; CHECK-NEXT: )
  (func $selectify-early-return (param $x i32) (result i32)
    (if
      (local.get $x)
      (return (local.get $x))
    )
    (i32.const 1)
  )

But obversely we expect something like this insted:

(func $selectify-early-return (param $x i32) (result i32)
  (select
    (local.get $0)
    (i32.const 1)
    (local.get $0)
  )
)

The main problem here: https://github.com/WebAssembly/binaryen/blob/main/src/passes/RemoveUnusedBrs.cpp#L1077
selectify just skip optimisation if ifFalse is not exists. So it is necessary to canonicalze such cases before applying selectify. Any idea how best to do this?

Start with the selectify logic near src/passes/RemoveUnusedBrs.cpp#L1077 and the existing cases in lit/remove-unused-brs.wast. Add the supplied early-return regression case and verify that the test expects the if without a false arm to be canonicalized so selectify can produce the shown select form.