avoid promote for homogeneous tuples

[nsajko]

According to Cthulhu's descend, promote doesn't get optimized out otherwise:

using CheckedSizeProduct
function f(t::Tuple{Int, Vararg{Int}})
    p = checked_size_product(t)
    if p isa Int
        p
    else
        throw(nothing)
    end
end
using Cthulhu: descend
descend(f, Tuple{Tuple{Vararg{Int, 15}}})

[nsajko]

Before this PR, on the main branch, the above script shows the following:

[...]
;  @ REPL[2]:2 within `f`
; ┌ @ /home/nsajko/src/gitlab.com/nsajko/CheckedSizeProduct/src/CheckedSizeProduct.jl:70 within `checked_size_product`
   %5 = getelementptr inbounds [15 x i64], ptr %0, i64 0, i64 4
   %6 = getelementptr inbounds [15 x i64], ptr %0, i64 0, i64 8
   %7 = getelementptr inbounds [15 x i64], ptr %0, i64 0, i64 12
   %8 = getelementptr inbounds [15 x i64], ptr %0, i64 0, i64 14
; │┌ @ promotion.jl:416 within `promote`
    %9 = load <4 x i64>, ptr %0, align 8
    store <4 x i64> %9, ptr %1, align 8
    %10 = getelementptr inbounds [15 x i64], ptr %1, i64 0, i64 4
    %11 = load <4 x i64>, ptr %5, align 8
    store <4 x i64> %11, ptr %10, align 8
    %12 = getelementptr inbounds [15 x i64], ptr %1, i64 0, i64 8
    %13 = load <4 x i64>, ptr %6, align 8
    store <4 x i64> %13, ptr %12, align 8
    %14 = getelementptr inbounds [15 x i64], ptr %1, i64 0, i64 12
    %15 = load <2 x i64>, ptr %7, align 8
    store <2 x i64> %15, ptr %14, align 8
    %16 = getelementptr inbounds [15 x i64], ptr %1, i64 0, i64 14
    %17 = load i64, ptr %8, align 8
    store i64 %17, ptr %16, align 8
; │└
; │ @ /home/nsajko/src/gitlab.com/nsajko/CheckedSizeProduct/src/CheckedSizeProduct.jl:71 within `checked_size_product`
[...]