Reorder Value and User fields to save 8 bytes of padding on 64-bit

Reviewered by: rafael

Differential Revision: http://reviews.llvm.org/D4073

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210501 91177308-0d34-0410-b5e6-96231b3b80d8