@inbounds in copyto! for structured broadcasting

[jishnub]

This seems to cut down the runtime of D .* D by half for 5000x5000 diagonal matrices. Using nightly v"1.10.0-DEV.450"

julia> using LinearAlgebra, BenchmarkTools

julia> D = Diagonal(rand(5000));

julia> @btime $D * $D;
  3.648 μs (2 allocations: 39.11 KiB)

julia> @btime $D .* $D; # ideally, should be identical to matrix multiplication
  8.729 μs (2 allocations: 39.11 KiB)

julia> B = Broadcast.instantiate(Broadcast.Broadcasted(*, (D, D)));

julia> @btime Base.copyto!($(copy(D)), $B); # the main method called
  8.390 μs (0 allocations: 0 bytes)

julia> function copyto2!(dest::Diagonal, bc::Broadcast.Broadcasted{<:LinearAlgebra.StructuredMatrixStyle})
           !LinearAlgebra.isstructurepreserving(bc) && !LinearAlgebra.fzeropreserving(bc) && return copyto!(dest, convert(Broadcast.Broadcasted{Nothing}, bc))
           axs = axes(dest)
           axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
           @inbounds for i in axs[1]
               dest.diag[i] = Broadcast._broadcast_getindex(bc, CartesianIndex(i, i))
           end
           return dest
       end
copyto2! (generic function with 1 method)

julia> @btime copyto2!($(copy(D)), $B);
  4.207 μs (0 allocations: 0 bytes)

Given that the axes are verified to be identical in the previous line, this seems safe.

[N5N3]

Since default copyto! has @inbounds I think there would be no objection if this helps performance.
But perhaps we should add @inbounds to other copyto! in this file?

[vtjnash]

These seem like a very large chunk of code to entirely mark @inbounds, since I think I count at least 6 different indexing operations in each of these. Can we mark just the operation that matters for it?

[jishnub]

I've updated it to only mark the broadcasted getindex calls, as these are the annotations that seem to matter in most cases. For the Lower and Upper triangular cases, both the getindex and setindex! calls improve performance.

[ViralBShah]

good to merge?

[jishnub]

This should be ready