added basic dataflow analysis to skslc

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405383003

Review-Url: https://codereview.chromium.org/2405383003
diff --git a/src/gpu/effects/GrXfermodeFragmentProcessor.cpp b/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
index 051061f..fba4050 100644
--- a/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
+++ b/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
@@ -98,10 +98,10 @@
     }
 
     // declare outputColor and emit the code for each of the two children
-    SkString srcColor("src");
+    SkString srcColor("xfer_src");
     this->emitChild(0, inputColor, &srcColor, args);
 
-    SkString dstColor("dst");
+    SkString dstColor("xfer_dst");
     this->emitChild(1, inputColor, &dstColor, args);
 
     // emit blend code