IRgen/ABI/ARM: Trust the backend to pass vectors correctly for the given ABI.

 - Therefore, we can lower out the NEON wrapper structs and pass the vectors
   directly. This makes a huge difference in the cleanliness of the IR after
   optimization.
 - I will trust, but verify, via future ABITest testing (for APCS-GNU, at
   least).

llvm-svn: 114618