Invalid optimization to select with --ignore-implicit-traps

使用提供的 WAT 模块和 -all -O1 --ignore-implicit-traps 重现该问题，然后检查将条件 struct.get 转换为 select 的优化。完成的标准是，优化器不再使可能触发 trap 的代码无条件执行，同时保留有效的优化后行为。

Optimizing this program with -all -O1 --ignore-implicit-traps:

(module
 (type $box (struct (field (mut i32)) (field (mut i64))))
 (export "f" (func $f))
 (func $f (param $0 (ref null $box)) (result i64)
  (block $label$1
   (block $label$2
    (br_if $label$2
     (ref.is_null
      (local.get $0)
     )
    )
    (br_if $label$1
     (i64.gt_s
      (struct.get $box 1
       (local.get $0)
      )
      (i64.const 8)
     )
    )
   )
   (return
    (i64.const 8)
   )
  )
  (struct.get $box 1
   (local.get $0)
  )
 )
)

yields

(module
 (type $box (struct (field (mut i32)) (field (mut i64))))
 (type $ref?|$box|_=>_i64 (func (param (ref null $box)) (result i64)))
 (export "f" (func $f))
 (func $f (param $0 (ref null $box)) (result i64)
  (if
   (select
    (ref.is_null
     (local.get $0)
    )
    (i32.const 1)
    (i64.gt_s
     (struct.get $box 1
      (local.get $0)
     )
     (i64.const 8)
    )
   )
   (return
    (i64.const 8)
   )
  )
  (struct.get $box 1
   (local.get $0)
  )
 )
)

Even when assuming that no traps are executed, it is invalid in general to change potentially trapping code from being conditionally executed to being unconditionally executed. This happens with the first struct.get here, as it is placed as the second arm of the select.