[ARM64-BE] Correctly deal with single element HFAs in varargs.

Just because the first "if" didn't fire, doesn't mean we can not have
an HFA in the "else" clause.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208216 91177308-0d34-0410-b5e6-96231b3b80d8