ARM ABI: simplify decisions on whether args can be expanded.

Homogeneous aggregates on AAPCS_VFP ARM need to be passed *without* being
flattened (e.g. [2 x float] rather than "float, float") for various weird ABI
reasons. However, this isn't the case for anything else; further, we know at
the ABIArgInfo::getDirect callsites whether this flattening is allowed.

So, we can get more unified ARM code, with a simpler Clang, by just using that
knowledge directly.

llvm-svn: 221559