Make map() and broadcast() faster for Union{T, Missing/Nothing} eltyp…

…es (#25828)

Storing typeof(el)/eltype(B) in a variable in the hot part of the loop kills
performance because despite being inferred as equal to Union{Type{T}, Type{Missing/Nothing}},
it is marked as ::Any. Only using typeof()/eltype() when isa() fails works around the problem
and makes map/broadcast(x->x, ::Vector{Union{Int, Missing}}) about 10 times faster.

base/array.jl

@@ -573,12 +573,11 @@ function collect_to!(dest::AbstractArray{T}, itr, offs, st) where T
     i = offs
     while !done(itr, st)
         el, st = next(itr, st)
-        S = typeof(el)
-        if S === T || S <: T
+        if el isa T || typeof(el) === T
             @inbounds dest[i] = el::T
             i += 1
         else
-            R = promote_typejoin(T, S)
+            R = promote_typejoin(T, typeof(el))
             new = similar(dest, R)
             copyto!(new,1, dest,1, i-1)
             @inbounds new[i] = el

base/broadcast.jl

@@ -500,14 +500,13 @@ end
             @nexprs $nargs i->(@inbounds val_i = _broadcast_getindex(A_i, I_i))
             # call the function
             V = @ncall $nargs f val
-            S = typeof(V)
             # store the result
-            if S <: eltype(B)
+            if V isa eltype(B)
                 @inbounds B[I] = V
             else
                 # This element type doesn't fit in B. Allocate a new B with wider eltype,
                 # copy over old values, and continue
-                newB = Base.similar(B, promote_typejoin(eltype(B), S))
+                newB = Base.similar(B, promote_typejoin(eltype(B), typeof(V)))
                 for II in Iterators.take(iter, count)
                     newB[II] = B[II]
                 end