Relax eltype condition in rdiv! with Diagonal

[ArunS-tack]

Closes JuliaLang/LinearAlgebra.jl#849.

Before
julia> rdiv!(ones(2,2), 3I(2)) ERROR: MethodError: no method matching rdiv!(::Matrix{Float64}, ::Diagonal{Int64, Vector{Int64}})

After
julia> rdiv!(ones(2,2), 3I(2)) 2×2 Matrix{Float64}: 0.333333 0.333333 0.333333 0.333333

[dkarrasch]

Maybe this is an opportunity to remove even more of the same-eltype conditions related to Diagonals? The same pattern can be seen in a few ldiv! and rdiv! methods below and above the current changes. I think it's safe to remove them. The functions will complain anyway if the target eltype cannot store the result type of the division.

[ArunS-tack]

I deleted the main repository for this PR since you had reviewed and approved the changes (mainly because it was made from master and not a sub-branch). I can open another PR to implement what you suggested.

[dkarrasch]

I deleted the main repository for this PR since you had reviewed and approved the changes (mainly because it was made from master and not a sub-branch). I can open another PR to implement what you suggested.

Sounds good.