added basic dataflow analysis to skslc

src/sksl/SkSLSPIRVCodeGenerator.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
  
 #include "SkSLSPIRVCodeGenerator.h"
 
 #include "string.h"
@@ skipping 2511 matching lines @@
     SpvId body = this->nextId();
     SpvId next = this->nextId();
     fContinueTarget.push(next);
     SpvId end = this->nextId();
     fBreakTarget.push(end);
     this->writeInstruction(SpvOpBranch, header, out);
     this->writeLabel(header, out);
     this->writeInstruction(SpvOpLoopMerge, end, next, SpvLoopControlMaskNone, out);
     this->writeInstruction(SpvOpBranch, start, out);
     this->writeLabel(start, out);
-    SpvId test = this->writeExpression(*f.fTest, out);
-    this->writeInstruction(SpvOpBranchConditional, test, body, end, out);
+    if (f.fTest) {
+        SpvId test = this->writeExpression(*f.fTest, out);
+        this->writeInstruction(SpvOpBranchConditional, test, body, end, out);
+    }
     this->writeLabel(body, out);
     this->writeStatement(*f.fStatement, out);
     if (fCurrentBlock) {
         this->writeInstruction(SpvOpBranch, next, out);
     }
     this->writeLabel(next, out);
     if (f.fNext) {
         this->writeExpression(*f.fNext, out);
     }
     this->writeInstruction(SpvOpBranch, header, out);
@@ skipping 102 matching lines @@
     this->writeWord(SpvVersion, out);
     this->writeWord(SKSL_MAGIC, out);
     std::stringstream buffer;
     this->writeInstructions(program, buffer);
     this->writeWord(fIdCount, out);
     this->writeWord(0, out); // reserved, always zero
     out << buffer.str();
 }
 
 }