Fix PPC ABI for ByVal structs with vector members

When a structure is passed by value, and that structure contains a vector
member, according to the PPC ABI, the structure will receive enhanced alignment
(so that the vector within the structure will always be aligned).

This should resolve PR16641.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190636 91177308-0d34-0410-b5e6-96231b3b80d8