[BUG] `simd.reduce_*` have inconsistent/wrong semantics when `size_out > 1`

[soraros]

Bug description

reduce_max is reduce-splat when size == size_out == 2, and no opt when size == size_out > 2. It's caused by the special-casing in simd.reduce: https://github.com/modularml/mojo/blob/6d2a7b552769358ec68a94b8fd1f6c2126d59ad9/stdlib/src/builtin/simd.mojo#L1766-L1767

I could have opened a PR, but I don't know if it's the intended/correct behaviour (I'd argue that it is not).

In fact, I think reduce can be simplified.

@always_inline
fn reduce[
    func: fn[type: DType, width: Int] (
        SIMD[type, width], SIMD[type, width]
    ) capturing -> SIMD[type, width],
    size_out: Int = 1,
](self) -> SIMD[type, size_out]:
    constrained[size_out <= size]()

    @parameter
    if size == size_out:  # size == 1 covered by this case since 0 < size_out <= size
        return rebind[SIMD[type, size_out]](self)
    # size > size_out
    alias half_size: Int = size // 2  # half_size >= size_out
    var lhs = self.slice[half_size, offset=0]()
    var rhs = self.slice[half_size, offset=half_size]()
    return func[type, half_size](lhs, rhs).reduce[func, size_out]()  # size decreases, so it will terminate

Steps to reproduce

fn main():
  alias type = DType.float32
  var a = SIMD[type, 2](0, 1)
  var b = SIMD[type, 4](0, 1, 2, 3)
  var c = SIMD[type, 8](0, 1, 2, 3, 4, 5, 6, 7)

  print(a.reduce_max[a.size]())  # [1.0, 1.0]
  print(b.reduce_max[b.size]())  # [0.0, 1.0, 2.0, 3.0]
  print(c.reduce_max[c.size]())  # [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0]

System information

Mojo 24.2 on Docker, Intel Mac