General reductions on host

Benchmark results before these changes (default build): tnl-benchmark-blas.baseline.html

Benchmark results before these changes (build with --optimize-vector-host-operations=true): tnl-benchmark-blas.host_unroll_prefetch.html

Benchmark results after the changes (default build): tnl-benchmark-blas.final.html

CC @oberhuber @hanousek