Implement compile-time optimization for cross().

We need to be careful to distinguish cross(vec3, vec3)--a real built-in
intrinsic--from cross(vec2, vec2)--an inline function in sksl_gpu, but
not actually a legitimate GLSL intrinsic.

Change-Id: I7e78c99dadfcbb637ae55a2503acfb7e591c932e
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413440
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed