Implement compile-time optimization for atan().

This is one of the few compile-time constant functions with a variable
number of arguments, but it's otherwise pretty normal as intrinsics go.

$genType  atan($genType  y, $genType  x);
$genHType atan($genHType y, $genHType x);
$genType  atan($genType  y_over_x);
$genHType atan($genHType y_over_x);

Change-Id: Ie852e10f37d73d53f69e806550872bc015f802d6
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413439
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
5 files changed