Handle "homogeneous aggregates" as required by the ARM AAPCS-VFP ABI.

A homogeneous aggregate is an aggregate data structure where after flattening
any nesting there are 1 to 4 elements of the same base type that is either a
float, double, or Neon vector.  All Neon vectors of the same size, either 64
or 128 bits, are treated as equivalent for this purpose.  When using the
AAPCS-VFP ABI, check for homogeneous aggregates and pass them as arguments by
expanding them into a sequence of their base types.  This requires extending
the existing support for expanded arguments to handle not only structs, but
also constant arrays and complex types.

llvm-svn: 136767