performance regressions introduced by #42766

[aviatesk]

@vtjnash detected 1510eaa causes performance regressions, which appear as increased allocations.
I reduce them to:

using BenchmarkTools

include("$(ENV["JULIA_PKG_DEVDIR"])/BaseBenchmarks/src/utils/RandUtils.jl")
using .RandUtils

_abs(x) = abs(x)
_abs(::Nothing) = nothing

const VEC_LENGTH = 10000
T = Bool
S = T
X = Vector{Union{T, Nothing}}(Vector{T}(RandUtils.samerand(S, VEC_LENGTH)));
@benchmark broadcast(identity, $X)
@benchmark broadcast(_abs, $X)

From the output of @code_typed broadcast(identity, X), it seems like 1510eaa fails to resolve Base.Broadcast.copyto_nonleaf!(...) statically somehow.