[spirv] Simpler logic for finding first BB in function. (#1936)

tools/clang/lib/SPIRV/SpirvFunction.cpp

@@ -37,14 +37,11 @@ bool SpirvFunction::invokeVisitor(Visitor *visitor, bool reverseOrder) {
     }).visit(basicBlocks.front());
   }
 
+  SpirvBasicBlock *firstBB = orderedBlocks.empty() ? nullptr : orderedBlocks[0];
+
   if (reverseOrder)
     std::reverse(orderedBlocks.begin(), orderedBlocks.end());
 
-  SpirvBasicBlock *firstBB =
-      orderedBlocks.empty()
-          ? nullptr
-          : reverseOrder ? orderedBlocks.back() : orderedBlocks[0];
-
   for (auto *bb : orderedBlocks) {
     // The first basic block of the function should first visit the function
     // variables.