performance dissipation due to avoidable FPU-divides (Feature #2488)

Optimization of unneeded slow FPU operations (SQRT, FDIV).
a/d; b/d; ==> a*(1/d); b*(1/d);
dbl/2 ==> dbl*0.5;
sqrt(a) < fabs(b) ==> a < b*b,
sqrt(a) < sqrt(b) ==> a < b,
log(1/sqrt(x)) ==> -log(x)*0.5,

retry: only relevant changes