Use do+while instead of while for loops which obviously have a

Use do+while instead of while for loops which obviously have a
non-zero trip count. Use SmallVector's pop_back_val().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92734 91177308-0d34-0410-b5e6-96231b3b80d8