Fix setindex! on Bidiagonal with ev zero bug.

[mcognetta]

Below is a MWE and explanation of the bug. The bug is that we can assign to structural zeros in Bidiagonal matrices when the B.ev vector is zero, but they show up in the off diagonal. For example:

julia> using LinearAlgebra

julia> B = Bidiagonal(rand(5), zeros(4), 'U')
5×5 Bidiagonal{Float64,Array{Float64,1}}:
 0.73714  0.0        ⋅         ⋅         ⋅      
  ⋅       0.800223  0.0        ⋅         ⋅      
  ⋅        ⋅        0.328117  0.0        ⋅      
  ⋅        ⋅         ⋅        0.519735  0.0     
  ⋅        ⋅         ⋅         ⋅        0.313492

julia> istril(B)
true

julia> istriu(B)
true

julia> B[2,1] = 3 # Note, this is in the lower off-diagonal but this is an upper Bidiagonal matrix
3

julia> B
5×5 Bidiagonal{Float64,Array{Float64,1}}:
 0.73714  3.0        ⋅         ⋅         ⋅      
  ⋅       0.800223  0.0        ⋅         ⋅      
  ⋅        ⋅        0.328117  0.0        ⋅      
  ⋅        ⋅         ⋅        0.519735  0.0     
  ⋅        ⋅         ⋅         ⋅        0.313492

julia> B[1,2] = 0
0

julia> B[4,5] = 10
10

julia> B
5×5 Bidiagonal{Float64,Array{Float64,1}}:
 0.73714  0.0        ⋅         ⋅          ⋅      
  ⋅       0.800223  0.0        ⋅          ⋅      
  ⋅        ⋅        0.328117  0.0         ⋅      
  ⋅        ⋅         ⋅        0.519735  10.0     
  ⋅        ⋅         ⋅         ⋅         0.313492

Why it happens: In setindex! are checking istriu(B) which is

julia/stdlib/LinearAlgebra/src/bidiag.jl

Line 249 in 802e719

istriu(M::Bidiagonal) = M.uplo == 'U' || iszero(M.ev)

So, when the Bidiagonal matrix has uplo = 'U' but B.ev is zero, setindex! allows us to set values on the lower off diagonal.

This fix has the added effect of improving access time in the (admittedly, artificial) pessimal case of the very last element in B.ev being the only non-zero one.

[StefanKarpinski]

@andreasnoack, any chance you could review?

[mcognetta]

If you do review, please check #32329 as well (they are both small and very similar).

[dkarrasch]

LGTM