Improve efficiency of minimum/maximum(::Diagonal) (#30236)

```
julia> DM = Diagonal(rand(100));

julia> @Btime minimum($DM);    # before
  27.987 μs (0 allocations: 0 bytes)

julia> @Btime minimum($DM);    # after
  246.091 ns (0 allocations: 0 bytes)
```

Author: jlapeyre

stdlib/LinearAlgebra/src/diagonal.jl

@@ -161,6 +161,18 @@ end
     r
 end
 
+function Base.minimum(D::Diagonal{T}) where T <: Number
+    mindiag = minimum(D.diag)
+    size(D, 1) > 1 && return (min(zero(T), mindiag))
+    return mindiag
+end
+
+function Base.maximum(D::Diagonal{T}) where T <: Number
+    maxdiag = Base.maximum(D.diag)
+    size(D, 1) > 1 && return (max(zero(T), maxdiag))
+    return maxdiag
+end
+
 @inline function getindex(D::Diagonal, i::Int, j::Int)
     @boundscheck checkbounds(D, i, j)
     if i == j

stdlib/LinearAlgebra/test/diagonal.jl

@@ -108,6 +108,12 @@ Random.seed!(1)
         for func in (det, tr)
             @test func(D) ≈ func(DM) atol=n^2*eps(relty)*(1+(elty<:Complex))
         end
+
+        if eltype(D) <: Real
+            @test minimum(D) ≈ minimum(DM)
+            @test maximum(D) ≈ maximum(DM)
+        end
+
         if relty <: BlasFloat
             for func in (exp, cis, sinh, cosh, tanh, sech, csch, coth)
                 @test func(D) ≈ func(DM) atol=n^3*eps(relty)