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

1 files changed, 7 insertions, 1 deletions

diff --git a/src/core/SkNx.h b/src/core/SkNx.h
index ed939c3e4f..af7194840a 100644
--- a/src/core/SkNx.h
+++ b/src/core/SkNx.h
@@ -68,7 +68,11 @@ public:
     SkNi operator << (int bits) const { return SkNi(fLo << bits, fHi << bits); }
     SkNi operator >> (int bits) const { return SkNi(fLo >> bits, fHi >> bits); }
 
-    // TODO: comparisons, min, max?
+    static SkNi Min(const SkNi& a, const SkNi& b) {
+        return SkNi(SkNi<N/2, T>::Min(a.fLo, b.fLo), SkNi<N/2, T>::Min(a.fHi, b.fHi));
+    }
+
+    // TODO: comparisons, max?
 
     template <int k> T kth() const {
         SkASSERT(0 <= k && k < N);
@@ -183,6 +187,8 @@ public:
     SkNi operator << (int bits) const { return SkNi(fVal << bits); }
     SkNi operator >> (int bits) const { return SkNi(fVal >> bits); }
 
+    static SkNi Min(const SkNi& a, const SkNi& b) { return SkNi(SkTMin(a.fVal, b.fVal)); }
+
     template <int k> T kth() const {
         SkASSERT(0 == k);
         return fVal;