Broadcast performance: fix type stability (#26942)
Remove a needless construction of a covariant typevar within a tuple. Tuples already are covariant.
Before:
```julia
julia> xset= 10:15
10:15
julia> yset= 12:14
12:14
julia> f(x,y) = sqrt( x^2 + y^2 )
f (generic function with 1 method)
julia> function loop3(xset, yset)
m = Matrix{Float64}(undef, 3, length(xset)*length(yset))
i = 1
@inbounds for x in xset, y in yset
m[:,i] .= (Float64(x), Float64(y), f(x,y))
i += 1
end
m
end
loop3 (generic function with 1 method)
julia> using BenchmarkTools
julia> @benchmark loop3($xset, $yset)
BenchmarkTools.Trial:
memory estimate: 5.59 KiB
allocs estimate: 109
--------------
minimum time: 9.634 μs (0.00% GC)
median time: 10.504 μs (0.00% GC)
mean time: 16.472 μs (32.25% GC)
maximum time: 43.241 ms (99.95% GC)
--------------
samples: 10000
evals/sample: 1
```
After:
```julia
julia> @benchmark loop3($xset, $yset)
BenchmarkTools.Trial:
memory estimate: 544 bytes
allocs estimate: 1
--------------
minimum time: 186.732 ns (0.00% GC)
median time: 193.974 ns (0.00% GC)
mean time: 217.001 ns (6.63% GC)
maximum time: 69.051 μs (99.42% GC)
--------------
samples: 10000
evals/sample: 616
```
