speed up `hvcat_fill!` by unrolling internal iteration for type-stability (#61426)

```
julia> using BenchmarkTools

julia> @btime [1.0 2; 3 4.0];
  75.248 ns (8 allocations: 336 bytes) # master
  9.593 ns (2 allocations: 112 bytes) # PR

julia> @btime [1 2 3im; 4 5 6im;;;]
  104.256 ns (10 allocations: 720 bytes) # master
  14.236 ns (2 allocations: 176 bytes) # PR
```

replaces https://github.com/JuliaLang/julia/pull/52028 with suggestion
given in
https://github.com/JuliaLang/julia/pull/52028#discussion_r1622850084