Avoid using Loop::getSubLoopsVector.

Passes should never modify it, just use the const version. While there
reduce copying in LoopInterchange. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242041 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3807,7 +3807,7 @@ bool LoopVectorizationLegality::canVectorize() {
   }
 
   // We can only vectorize innermost loops.
-  if (!TheLoop->getSubLoopsVector().empty()) {
+  if (!TheLoop->empty()) {
     emitAnalysis(VectorizationReport() << "loop is not the innermost loop");
     return false;
   }