fix PR7742 / rdar://8250764, a miscompilation of struct

fix PR7742 / rdar://8250764, a miscompilation of struct
return where the struct has a base but no fields.  This
was because the x86-64 abi logic was checking the wrong
predicate in one place.

This was introduced in r91874, which was a fix for PR5831,
which lacked a CHECK line, so I verified and added it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109759 91177308-0d34-0410-b5e6-96231b3b80d8