fix fastpaths for materializing transposes / adjoints of sparse matrices

[KristofferC]

Setup:

using LinearAlgebra
using SparseArrays
using BenchmarkTools
n = 1000
A = sparse(1.0I,n,n);
B = sparse(1.0I,n,n);

Before PR:

julia> @btime $A + $B';
  5.307 ms (28 allocations: 112.11 KiB)

julia> @btime $A + $(B');
  5.305 ms (27 allocations: 112.09 KiB)

After PR:

julia> @btime $A + $B';
  13.897 μs (10 allocations: 63.52 KiB)

julia> @btime $A + $(B');
  13.852 μs (9 allocations: 63.50 KiB)

Fixes #28012

[martinholters]

Should probably relax the signature to Adjoint{<:Any,<:SparseMatrixCSC} (which is also the one of the copy method this should dispatch to); likewise below.

[KristofferC]

Hmm

julia> B = sprand(1,1,1.0);

julia> SparseMatrixCSC(M::Adjoint{<:Any,SparseMatrixCSC}) = copy(M)
SparseMatrixCSC

julia> @which SparseMatrixCSC(B')
(::Type{SparseMatrixCSC})(M::AbstractArray{Tv,2}) where Tv in SparseArrays at /Users/kristoffer/julia/usr/share/julia/stdlib/v0.7/SparseArrays/src/sparsematrix.jl:371

julia> SparseMatrixCSC(M::LinearAlgebra.Adjoint{Tv,SparseMatrixCSC{Tv,Ti}}) where {Tv,Ti} = copy(M)
SparseMatrixCSC

julia> @which SparseMatrixCSC(B')
(::Type{SparseMatrixCSC})(M::Adjoint{Tv,SparseMatrixCSC{Tv,Ti}}) where {Tv, Ti} in Main at REPL[7]:1

[martinholters]

Adjoint{<:Any,<:SparseMatrixCSC}, not Adjoint{<:Any,SparseMatrixCSC}:

julia> B = sprand(1,1,1.0);

julia> SparseMatrixCSC(M::Adjoint{<:Any,<:SparseMatrixCSC}) = copy(M)
SparseMatrixCSC

julia> @which SparseMatrixCSC(B')
(::Type{SparseMatrixCSC})(M::Adjoint{#s2,#s1} where #s1<:SparseMatrixCSC where #s2) in Main at REPL[19]:1

[KristofferC]

Oops, sorry.

[KristofferC]

Fixed.

[martinholters]

We might want to verify the return type, too, because the new definitions do not obviously return a SparseMatrixCSC.

[KristofferC]

Done

[Sacha0]

(Alternatively @test A' == SparseMatrix(A')::SparseMatrixCSC.)