x86-32 Darwin ABI: Handle small structures correctly.

 - Small structures are returned in a register if: 
    1. They fit nicely in a  register.
    2. All fields fit nicely in a register.
   (more or less)

 - We now pass the first 5000 ABITests if unions are disabled.

 - <rdar://problem/6497882> [irgen] x86-32 ABI compatibility with
   small structs

llvm-svn: 68197