[Vectorize] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/VPlanTransforms.h:31:1: error: class
  'VFRange' was previously declared as a struct; this is valid, but
  may result in linker errors under the Microsoft C++ ABI
  [-Werror,-Wmismatched-tags]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -28,7 +28,7 @@ class PredicatedScalarEvolution;
 class TargetLibraryInfo;
 class VPBuilder;
 class VPRecipeBuilder;
-class VFRange;
+struct VFRange;
 
 extern cl::opt<bool> VerifyEachVPlan;