Aggressive constprop in istriu/istril for structured matrices (#54437)
This makes the following evaluate at compile-time:
```julia
julia> U = UpperTriangular(rand(2,2));
julia> @code_typed istriu(U)
CodeInfo(
1 ─ return true
) => Bool
```
Also, this reduces latency in this operation:
```julia
julia> @time (U -> istriu(U))(U)
0.069995 seconds (158.88 k allocations: 8.715 MiB, 83.72% compilation time) # nightly
0.035610 seconds (156.62 k allocations: 8.594 MiB, 68.18% compilation time) # This PR
```
Similar methods are annotated for other structured matrix types, where
the results may be trivially obtained from the structure for certain
values of the band index `k`.
julia
25c8128d - Aggressive constprop in istriu/istril for structured matrices (#54437)
