Don't read destination indices when copying structured matrices (#55322)
Fixes the following regression introduced in v1.11
```julia
julia> using LinearAlgebra
julia> D = Diagonal(rand(4));
julia> T = Tridiagonal(Vector{BigFloat}(undef, 3), Vector{BigFloat}(undef, 4), Vector{BigFloat}(undef, 3))
4×4 Tridiagonal{BigFloat, Vector{BigFloat}}:
#undef #undef ⋅ ⋅
#undef #undef #undef ⋅
⋅ #undef #undef #undef
⋅ ⋅ #undef #undef
julia> copyto!(T, D)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex
@ ./essentials.jl:907 [inlined]
[2] _broadcast_getindex
@ ./broadcast.jl:644 [inlined]
[3] _getindex
@ ./broadcast.jl:675 [inlined]
[4] _broadcast_getindex
@ ./broadcast.jl:650 [inlined]
[5] getindex
@ ./broadcast.jl:610 [inlined]
[6] macro expansion
@ ./broadcast.jl:973 [inlined]
[7] macro expansion
@ ./simdloop.jl:77 [inlined]
[8] copyto!
@ ./broadcast.jl:972 [inlined]
[9] copyto!
@ ./broadcast.jl:925 [inlined]
[10] materialize!
@ ./broadcast.jl:883 [inlined]
[11] materialize!
@ ./broadcast.jl:880 [inlined]
[12] _copyto_banded!(T::Tridiagonal{BigFloat, Vector{BigFloat}}, D::Diagonal{Float64, Vector{Float64}})
@ LinearAlgebra ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/LinearAlgebra/src/special.jl:323
[13] copyto!(dest::Tridiagonal{BigFloat, Vector{BigFloat}}, src::Diagonal{Float64, Vector{Float64}})
@ LinearAlgebra ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/LinearAlgebra/src/special.jl:315
[14] top-level scope
@ REPL[4]:1
```
After this PR
```julia
julia> copyto!(T, D)
4×4 Tridiagonal{BigFloat, Vector{BigFloat}}:
0.909968 0.0 ⋅ ⋅
0.0 0.193341 0.0 ⋅
⋅ 0.0 0.194794 0.0
⋅ ⋅ 0.0 0.506905
```
The current implementation used an optimization that may not be
applicable for non-isbits types, and this PR ensures that we always read
from the source and write to the destination.
julia
fb4e4e5b - Don't read destination indices when copying structured matrices (#55322)
