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.ge_u
    (i32.load
     (i32.const 0)
    )
    (block (result i32)
     (i32.store
      (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: #7556