Handle functions with struct arguments or return types and the regparm

attribute. It is a variation of the x86_64 ABI:

* A struct returned indirectly uses the first register argument to pass the
  pointer.
* Floats, Doubles and structs containing only one of them are not passed in
  registers.
* Other structs are split into registers if they fit on the remaining ones.
  Otherwise they are passed in memory.
* When a struct doesn't fit it still consumes the registers.

llvm-svn: 161022