[LoopVectorize][NFC] Move "LV: Selecting VF" debug output (llvm#120744)

Move the debug output that prints out the selected VF from
selectVectorizationFactor -> computeBestVF. This means that the output
will still be written even after removing the assert for the legacy and
vplan cost models matching.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4695,7 +4695,6 @@ VectorizationFactor LoopVectorizationPlanner::selectVectorizationFactor() {
                  !isMoreProfitable(ChosenFactor, ScalarCost)) dbgs()
              << "LV: Vectorization seems to be not beneficial, "
              << "but was forced by a user.\n");
-  LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << ChosenFactor.Width << ".\n");
   return ChosenFactor;
 }
 #endif
@@ -7624,6 +7623,7 @@ VectorizationFactor LoopVectorizationPlanner::computeBestVF() {
          "when vectorizing, the scalar cost must be computed.");
 #endif
 
+  LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << BestFactor.Width << ".\n");
   return BestFactor;
 }