Implement compile-time optimization for step().

$genType step($genType edge, $genType x);
$genType step(float edge, $genType x);
$genHType step($genHType edge, $genHType x);
$genHType step(half edge, $genHType x);

Change-Id: If52634d7a247772e922cb3fd2e3e8a7c36c275f6
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411842
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
6 files changed