[PowerPC64] Fix passing of single-vector-member structs to match ABI.

The 64-bit PowerPC ELF ABI requires a struct that contains a single
vector member to be passed in a vector register as though the wrapping
struct were not present.  Instead we were passing this as a byval
struct.

The same logic was already present for floating-point arguments, so
this patch just extends the logic to handle vector types.  The new
test case verifies that clang coerces the parameter and annotates it
as inreg.

Thanks,
Bill

llvm-svn: 186993