optimization of trivial MulAddMap evaluate!

The (0,1) AttachDirichletMap is just (A,b,c) -> c .

So we can avoid `mul!` in this case (but keep the `copyto!` for type
stability when c is real but b and d=cache[2] are complex).

I believe this is safe regarding type stability since
`return_cache(::AttachDirichletMap)` uses an eager `Base.*` to create
the `cache`, it does not rely on this evaluate.

src/Arrays/AlgebraMaps.jl

@@ -35,6 +35,7 @@ function evaluate!(cache,k::MulAddMap,a,b,c)
   setsize!(cache,size(c))
   d = cache.array
   copyto!(d,c)
+  iszero(k.α) && isone(k.β) && return d
   mul!(d,a,b,k.α,k.β)
   d
 end

src/Fields/ArrayBlocks.jl

@@ -1281,6 +1281,7 @@ function evaluate!(cache,k::MulAddMap,a::ArrayBlock,b::ArrayBlock,c::ArrayBlock)
   _setsize_mul!(c1,a,b)
   d = evaluate!(c2,unwrap_cached_array,c1)
   copyto!(d,c)
+  iszero(k.α) && isone(k.β) && return d
   mul!(d,a,b,k.α,k.β)
   d
 end
@@ -1517,4 +1518,4 @@ function Base.show(io::IO,o::ArrayBlockView)
 end
 
 LinearAlgebra.diag(a::MatrixBlockView) = view(a.array.array, diag(a.block_map))
-LinearAlgebra.diag(a::MatrixBlock) = view(a.array,diag(CartesianIndices(a.array)))
+LinearAlgebra.diag(a::MatrixBlock) = view(a.array,diag(CartesianIndices(a.array)))