add Min to SkNi, specialized for u8 and u16 on SSE and NEON

0x8001 / 0x7fff don't seem to work, but we were close: 0x8000 does.

I plan to use this to implement the Difference xfermode,
and it seems generally handy.

BUG=skia:

Review URL: https://codereview.chromium.org/1133933004
4 files changed