Implement Metal polyfill for sign(genIType).

This uses nested selects to compute `(x > 0) ? 1 : (x < 0) ? -1 : 0`.

Change-Id: I1a87fc8506767bb0a9dd77ba2193b330e0a4d0a2
Bug: skia:12898, skia:11209
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503486
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed