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`.

stdlib/LinearAlgebra/src/bidiag.jl

@@ -309,7 +309,7 @@ end
 
 iszero(M::Bidiagonal) = iszero(M.dv) && iszero(M.ev)
 isone(M::Bidiagonal) = all(isone, M.dv) && iszero(M.ev)
-function istriu(M::Bidiagonal, k::Integer=0)
+Base.@constprop :aggressive function istriu(M::Bidiagonal, k::Integer=0)
     if M.uplo == 'U'
         if k <= 0
             return true
@@ -328,7 +328,7 @@ function istriu(M::Bidiagonal, k::Integer=0)
         end
     end
 end
-function istril(M::Bidiagonal, k::Integer=0)
+Base.@constprop :aggressive function istril(M::Bidiagonal, k::Integer=0)
     if M.uplo == 'U'
         if k >= 1
             return true

stdlib/LinearAlgebra/src/diagonal.jl

@@ -223,8 +223,8 @@ iszero(D::Diagonal) = all(iszero, D.diag)
 isone(D::Diagonal) = all(isone, D.diag)
 isdiag(D::Diagonal) = all(isdiag, D.diag)
 isdiag(D::Diagonal{<:Number}) = true
-istriu(D::Diagonal, k::Integer=0) = k <= 0 || iszero(D.diag) ? true : false
-istril(D::Diagonal, k::Integer=0) = k >= 0 || iszero(D.diag) ? true : false
+Base.@constprop :aggressive istriu(D::Diagonal, k::Integer=0) = k <= 0 || iszero(D.diag) ? true : false
+Base.@constprop :aggressive istril(D::Diagonal, k::Integer=0) = k >= 0 || iszero(D.diag) ? true : false
 function triu!(D::Diagonal{T}, k::Integer=0) where T
     n = size(D,1)
     if !(-n + 1 <= k <= n + 1)

stdlib/LinearAlgebra/src/hessenberg.jl

@@ -74,7 +74,7 @@ real(H::UpperHessenberg{<:Real}) = H
 real(H::UpperHessenberg{<:Complex}) = UpperHessenberg(triu!(real(H.data),-1))
 imag(H::UpperHessenberg) = UpperHessenberg(triu!(imag(H.data),-1))
 
-function istriu(A::UpperHessenberg, k::Integer=0)
+Base.@constprop :aggressive function istriu(A::UpperHessenberg, k::Integer=0)
     k <= -1 && return true
     return _istriu(A, k)
 end

stdlib/LinearAlgebra/src/triangular.jl

@@ -351,11 +351,11 @@ function Base.replace_in_print_matrix(A::Union{LowerTriangular,UnitLowerTriangul
     return i >= j ? s : Base.replace_with_centered_mark(s)
 end
 
-function istril(A::Union{LowerTriangular,UnitLowerTriangular}, k::Integer=0)
+Base.@constprop :aggressive function istril(A::Union{LowerTriangular,UnitLowerTriangular}, k::Integer=0)
     k >= 0 && return true
     return _istril(A, k)
 end
-function istriu(A::Union{UpperTriangular,UnitUpperTriangular}, k::Integer=0)
+Base.@constprop :aggressive function istriu(A::Union{UpperTriangular,UnitUpperTriangular}, k::Integer=0)
     k <= 0 && return true
     return _istriu(A, k)
 end

stdlib/LinearAlgebra/src/tridiag.jl

@@ -332,7 +332,7 @@ end
 
 # tril and triu
 
-function istriu(M::SymTridiagonal, k::Integer=0)
+Base.@constprop :aggressive function istriu(M::SymTridiagonal, k::Integer=0)
     if k <= -1
         return true
     elseif k == 0
@@ -341,7 +341,7 @@ function istriu(M::SymTridiagonal, k::Integer=0)
         return iszero(_evview(M)) && iszero(M.dv)
     end
 end
-istril(M::SymTridiagonal, k::Integer) = istriu(M, -k)
+Base.@constprop :aggressive istril(M::SymTridiagonal, k::Integer) = istriu(M, -k)
 iszero(M::SymTridiagonal) =  iszero(_evview(M)) && iszero(M.dv)
 isone(M::SymTridiagonal) =  iszero(_evview(M)) && all(isone, M.dv)
 isdiag(M::SymTridiagonal) =  iszero(_evview(M))
@@ -718,7 +718,7 @@ end
 
 iszero(M::Tridiagonal) = iszero(M.dl) && iszero(M.d) && iszero(M.du)
 isone(M::Tridiagonal) = iszero(M.dl) && all(isone, M.d) && iszero(M.du)
-function istriu(M::Tridiagonal, k::Integer=0)
+Base.@constprop :aggressive function istriu(M::Tridiagonal, k::Integer=0)
     if k <= -1
         return true
     elseif k == 0
@@ -729,7 +729,7 @@ function istriu(M::Tridiagonal, k::Integer=0)
         return iszero(M.dl) && iszero(M.d) && iszero(M.du)
     end
 end
-function istril(M::Tridiagonal, k::Integer=0)
+Base.@constprop :aggressive function istril(M::Tridiagonal, k::Integer=0)
     if k >= 1
         return true
     elseif k == 0