SkRasterPipelineBlitter: blitAntiH + full coverge --> blitH

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4442

Change-Id: Iaf3da7b82c1b3a41fe66c83fb5f9ca589f64851f
Reviewed-on: https://skia-review.googlesource.com/4442
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/src/core/SkRasterPipelineBlitter.cpp b/src/core/SkRasterPipelineBlitter.cpp
index 6aff8dd..b8c0107 100644
--- a/src/core/SkRasterPipelineBlitter.cpp
+++ b/src/core/SkRasterPipelineBlitter.cpp
@@ -234,9 +234,13 @@
 
     fDstPtr = fDst.writable_addr(0,y);
     for (int16_t run = *runs; run > 0; run = *runs) {
-        fConstantCoverage = *aa * (1/255.0f);
-        fBlitAntiH(x, run);
-
+        switch (*aa) {
+            case 0x00:                       break;
+            case 0xff: this->blitH(x,y,run); break;
+            default:
+                fConstantCoverage = *aa * (1/255.0f);
+                fBlitAntiH(x, run);
+        }
         x    += run;
         runs += run;
         aa   += run;