Broadcast binary ops involving strided triangular

[jishnub]

Currently, we evaluate expressions like (A::UpperTriangular) + (B::UpperTriangular) using broadcasting if both A and B have strided parents, and forward the summation to the parents otherwise. This PR changes this to use broadcasting if either of the two has a strided parent. This avoids accessing the parent corresponding to the structural zero elements, as the index might not be initialized.

Fixes #55590

This isn't a general fix, as we still sum the parents if neither is strided. However, it will address common cases.

This also improves performance, as we only need to loop over one half:

julia> using LinearAlgebra

julia> U = UpperTriangular(zeros(100,100));

julia> B = Bidiagonal(zeros(100), zeros(99), :U);

julia> @btime $U + $B;
  35.530 μs (4 allocations: 78.22 KiB) # nightly
  13.441 μs (4 allocations: 78.22 KiB) # This PR