Remove willReadDst from GrFragmentProcessor.

Since only XP's can read dst now, there is no reason to have this query on GrFP.
This also triggered a chain reaction of cleaning up/removing unnecessary code
elsewhere.

BUG=skia:

Review URL: https://codereview.chromium.org/851143003
diff --git a/src/gpu/GrOptDrawState.cpp b/src/gpu/GrOptDrawState.cpp
index 742ffad..bc1ce7d 100644
--- a/src/gpu/GrOptDrawState.cpp
+++ b/src/gpu/GrOptDrawState.cpp
@@ -89,6 +89,8 @@
     this->adjustProgramFromOptimizations(drawState, optFlags, colorPOI, coveragePOI,
                                          &firstColorStageIdx, &firstCoverageStageIdx);
 
+    fDescInfo.fReadsDst = fXferProcessor->willReadDstColor();
+
     bool usesLocalCoords = false;
 
     // Copy Stages from DS to ODS
@@ -124,31 +126,22 @@
                                                     const GrProcOptInfo& coveragePOI,
                                                     int* firstColorStageIdx,
                                                     int* firstCoverageStageIdx) {
-    fDescInfo.fReadsDst = false;
     fDescInfo.fReadsFragPosition = false;
 
     if ((flags & GrXferProcessor::kIgnoreColor_OptFlag) ||
         (flags & GrXferProcessor::kOverrideColor_OptFlag)) {
         *firstColorStageIdx = ds.numColorStages();
     } else {
-        fDescInfo.fReadsDst = colorPOI.readsDst();
         fDescInfo.fReadsFragPosition = colorPOI.readsFragPosition();
     }
 
     if (flags & GrXferProcessor::kIgnoreCoverage_OptFlag) {
         *firstCoverageStageIdx = ds.numCoverageStages();
     } else {
-        if (coveragePOI.readsDst()) {
-            fDescInfo.fReadsDst = true;
-        }
         if (coveragePOI.readsFragPosition()) {
             fDescInfo.fReadsFragPosition = true;
         }
     }
-
-    if (fXferProcessor->willReadDstColor()) {
-        fDescInfo.fReadsDst = true;
-    }
 }
 
 void GrOptDrawState::finalize(GrGpu* gpu) {