Optimize block tails where a dropped br_if's value is redundant

[xuruiyang2002]

If a block ends with a br_if followed by a value that is the same as the br_if's value (and has no side effects), the value of br_if and br_if itself are redundant and can be removed. For example:

(block $block (result i32)
 ..
 (drop
  (br_if $block
   (value)
   (condition)
  )
 )
 (value)
)

=>

(block $block (result i32)
 ..
 (drop
  (condition)
 )
 (value)
)

Fixes: #7489

[xuruiyang2002]

Thanks! Learned a lot from your comments and good coding taste. I've updated as you suggested.