added basic dataflow analysis to skslc

src/sksl/SkSLSPIRVCodeGenerator.cpp

Index: src/sksl/SkSLSPIRVCodeGenerator.cpp
diff --git a/src/sksl/SkSLSPIRVCodeGenerator.cpp b/src/sksl/SkSLSPIRVCodeGenerator.cpp
index 5403ba362814057ad46e2047d96a3f730b4a54c8..a4919686742f20c006b2a39f933e2aeb0184b228 100644
--- a/src/sksl/SkSLSPIRVCodeGenerator.cpp
+++ b/src/sksl/SkSLSPIRVCodeGenerator.cpp
@@ -2529,8 +2529,10 @@ void SPIRVCodeGenerator::writeForStatement(const ForStatement& f, std::ostream&
     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) {