Aggressive constprop in istriu/istril for structured matrices

[jishnub]

This makes the following evaluate at compile-time:

julia> U = UpperTriangular(rand(2,2));

julia> @code_typed istriu(U)
CodeInfo(
1 ─     return true
) => Bool

Also, this reduces latency in this operation:

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.