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Specialize indexing triangular matrices with BandIndex (#55644)
With this, certain indexing operations involving a `BandIndex` may be evaluated as constants. This isn't used directly presently, but might allow for more performant broadcasting in the future. With this, ```julia julia> n = 3; T = Tridiagonal(rand(n-1), rand(n), rand(n-1)); julia> @code_warntype ((T,j) -> UpperTriangular(T)[LinearAlgebra.BandIndex(2,j)])(T, 1) MethodInstance for (::var"#17#18")(::Tridiagonal{Float64, Vector{Float64}}, ::Int64) from (::var"#17#18")(T, j) @ Main REPL[12]:1 Arguments #self#::Core.Const(var"#17#18"()) T::Tridiagonal{Float64, Vector{Float64}} j::Int64 Body::Float64 1 ─ %1 = Main.UpperTriangular(T)::UpperTriangular{Float64, Tridiagonal{Float64, Vector{Float64}}} │ %2 = LinearAlgebra.BandIndex::Core.Const(LinearAlgebra.BandIndex) │ %3 = (%2)(2, j)::Core.PartialStruct(LinearAlgebra.BandIndex, Any[Core.Const(2), Int64]) │ %4 = Base.getindex(%1, %3)::Core.Const(0.0) └── return %4 ``` The indexing operation may be evaluated at compile-time, as the band index is constant-propagated.
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