Streamline branching in Jet copysign and fdim

- Pass the second argument g of copysign(f, g) by const reference
  instead of by value, and return (sa * sb) * f.v directly when g != 0
  to avoid evaluating 0 * g.v.
- Check isgreater(f, g) before isnan in fdim so that normal inputs with
  f > g short-circuit on the first comparison.

Benchmark (Apple M1 Pro):
----------------------------------------------------------------------
Benchmark                                     Before    After   Change
----------------------------------------------------------------------
BM_JetCopysign<3>                            2.27 ns 0.453 ns   -80.0%
BM_JetCopysign<6>                            2.33 ns 0.506 ns   -78.3%
BM_JetCopysign<12>                           4.08 ns  1.23 ns   -69.9%
BM_JetCopysign<30>                           9.76 ns  5.23 ns   -46.4%
BM_JetFdim<3>                                1.98 ns 0.728 ns   -63.2%
BM_JetFdim<6>                                2.07 ns  1.25 ns   -39.6%
BM_JetFdim<12>                               2.78 ns  1.83 ns   -34.2%
BM_JetFdim<30>                               5.28 ns  4.86 ns    -8.0%
----------------------------------------------------------------------

Change-Id: I3b4ceebad76cbb0eddc42be6368872a9e5543b71