Fix bug 14779 for passing anonymous aggregates [patch by Kai Nacke].

The PPC backend doesn't handle these correctly.  This patch uses logic
similar to that in the X86 and ARM backends to track these arguments
properly.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175635 91177308-0d34-0410-b5e6-96231b3b80d8