Suppress /RTCc error about losing information during case to smaller type

Bug: skia:
Change-Id: I6c83504918fc1ff71f9153d3a2b02fa4c2d70871
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201646
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/core/SkScan_AAAPath.cpp b/src/core/SkScan_AAAPath.cpp
index e5209a5..4925aa1 100644
--- a/src/core/SkScan_AAAPath.cpp
+++ b/src/core/SkScan_AAAPath.cpp
@@ -568,7 +568,7 @@
 
 // The alpha of right-triangle (a, a*b)
 static inline SkAlpha partialTriangleToAlpha(SkFixed a, SkFixed b) {
-    return partialTriangleToAlpha16(a, b) >> 8;
+    return (partialTriangleToAlpha16(a, b) >> 8) & 0xFF;
 }
 
 static inline SkAlpha getPartialAlpha(SkAlpha alpha, SkFixed partialHeight) {