minor Sk4px cleanup

I've removed a bunch of unused code,
inlined routines that are only used once,
removed platform code where a portable code works fine,
and renamed methods and updated comments.

I'm thinking about getting rid of Sk4px entirely, and boiling it down
instead to a couple helper methods on Sk16b/Sk16h.  This is a start.

Change-Id: I35b6c44710aa2cefe8c1e07fc2cb877a4042e8a5
Reviewed-on: https://skia-review.googlesource.com/c/178985
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
diff --git a/tests/SkNxTest.cpp b/tests/SkNxTest.cpp
index 7afedfe..f210974 100644
--- a/tests/SkNxTest.cpp
+++ b/tests/SkNxTest.cpp
@@ -185,8 +185,8 @@
         int exact = (a*b+127)/255;
 
         // Duplicate a and b 16x each.
-        auto av = Sk4px::DupAlpha(a),
-             bv = Sk4px::DupAlpha(b);
+        Sk4px av = Sk16b(a),
+              bv = Sk16b(b);
 
         // This way should always be exactly correct.
         int correct = (av * bv).div255()[0];
@@ -202,22 +202,6 @@
     }
 }
 
-DEF_TEST(Sk4px_widening, r) {
-    SkPMColor colors[] = {
-        SkPreMultiplyColor(0xff00ff00),
-        SkPreMultiplyColor(0x40008000),
-        SkPreMultiplyColor(0x7f020406),
-        SkPreMultiplyColor(0x00000000),
-    };
-    auto packed = Sk4px::Load4(colors);
-
-    auto wideLo = packed.widenLo(),
-         wideHi = packed.widenHi(),
-         wideLoHi    = packed.widenLoHi(),
-         wideLoHiAlt = wideLo + wideHi;
-    REPORTER_ASSERT(r, 0 == memcmp(&wideLoHi, &wideLoHiAlt, sizeof(wideLoHi)));
-}
-
 DEF_TEST(SkNx_abs, r) {
     auto fs = Sk4f(0.0f, -0.0f, 2.0f, -4.0f).abs();
     REPORTER_ASSERT(r, fs[0] == 0.0f);