Use fallthrough in optimizeInstructions to further optimize (unsigned)x < 0 ==> i32(0)

[xuruiyang2002]

Currently, wasm-opt cannot optimize the code as expected: (unsigned)x < 0 ==> i32(0).

(i32.lt_u
 (i32.load $0 (i32.const 0) )
 (block (result i32)
  (i32.store $0
   (i32.const 0)
   (i32.const 0)
  )
  (i32.const 0)
 )
)

It should have been optimized after merge-blocks, as the store could have been hoisted. However, the load and store cannot be reordered, preventing wasm-opt from doing so.

This can be addressed in optimizeInstructions by using fallthrough to hoist the constant zero, enabling further optimizations.

Fixes: #7455