Improve the comment from r185794 (re: PromoteIntRes_BUILD_VECTOR)

In response to Duncan's review, I believe that the original comment was not as
clear as it could be. Hopefully, this is better.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185824 91177308-0d34-0410-b5e6-96231b3b80d8