mat: rewrite Cholesky.ToSym without calling SymOuterK
Created by: vladimir-ch
A substitute for the Dlauum attempt. Cholesky.ToSym
is not super important but with this PR it becomes slightly faster.
name old time/op new time/op delta
CholeskyToSym/n=10-4 2.65µs ± 2% 0.78µs ± 0% -70.75% (p=0.029 n=4+4)
CholeskyToSym/n=100-4 329µs ± 1% 124µs ± 0% -62.33% (p=0.029 n=4+4)
CholeskyToSym/n=1000-4 241ms ± 2% 175ms ± 0% -27.34% (p=0.029 n=4+4)
Please take a look.