pass argument vectors in a type that corresponds to the user type if

pass argument vectors in a type that corresponds to the user type if
possible.  This improves the example to pass <4 x float> instead of
<2 x double> but we still get awful code, and still don't get the
return value right.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109700 91177308-0d34-0410-b5e6-96231b3b80d8