Add an internal helper function to do more in-place updating

Currently the `∇(x̄, f, Arg{N}, args...)` method, which updates `x̄` with
the result of `∇(f, Arg{N}, args...)`. This is done in-place for some
functions `f` but not all. In the case of the fallback method, we can
use dispatch to determine whether it's safe to do this in-place.

src/core.jl

@@ -175,7 +175,14 @@ output and `ȳ` the reverse-mode sensitivity of `y`.
 ∇(y::Node, ȳ) = propagate(tape(y), reverse_tape(y, ȳ))
 @inline ∇(y::Node{<:∇Scalar}) = ∇(y, one(unbox(y)))
 
-@inline ∇(x̄, f, ::Type{Arg{N}}, args...) where N = x̄ + ∇(f, Arg{N}, args...)
+# This is a fallback method where we don't necessarily know what we'll be adding and whether
+# we can update the value in-place, so we'll try to be clever and dispatch.
+@inline ∇(x̄, f, ::Type{Arg{N}}, args...) where {N} = update!(x̄, ∇(f, Arg{N}, args...))
+# Use broadcast for mixed array/scalar operations
+@inline update!(x̄::∇Scalar, y::∇Array) = x̄ .+ y
+@inline update!(x̄::∇Array, y::∇ArrayOrScalar) = (x̄ .= x̄ .+ y)
+# Otherwise use regular addition
+@inline update!(x̄, y) = x̄ + y
 
 """
     ∇(f; get_output::Bool=false)