basic: Add volatile to prevent optimizations

Prevent the compiler from optimizing away initialization loops

test_conformance/basic/test_vector_swizzle.cpp

@@ -511,7 +511,7 @@ static void makeReference(std::vector<T>& ref)
     ref.resize(refSize);
     std::fill(ref.begin(), ref.end(), 99);
 
-    size_t dstIndex = 0;
+    volatile size_t dstIndex = 0;
 
     // single channel lvalue
     for (size_t i = 0; i < N; i++)