Reuse du2 of Tridiagonal matrix for pivoting in lu! if extant

[brenhinkeller]

The LinearAlgebra.Tridiagonal type contains an optional upper second diagonal for pivoting

julia> using LinearAlgebra

julia> a = Tridiagonal(ones(9), ones(10), ones(9));

julia> isdefined(a, :du2)
false

julia> b = Tridiagonal(ones(9), ones(10), ones(9), ones(8));

julia> isdefined(b, :du2)
true

However lu! currently does not use this, and instead just allocates a new one even if it already exists.

This PR has lu!(A::Tridiagonal, ...) check if the second upper diagonal is already defined and of the proper size and type, and uses it if so.

This can be a nontrivial speedup and reduction in allocations if lu! is used in a hot loop.

[brenhinkeller]

Oh, or should this be opened against https://github.com/JuliaLang/LinearAlgebra.jl instead?
edit: looks like this is the right place for now