[IRVerifier] Allow StructRet in statepoint

Summary:
StructRet attribute is not allowed in vararg calls. The statepoint
intrinsic is vararg, but the wrapped function may be not. Allow
calls of statepoint with StructRet arg, as long as the wrapped
function is not vararg.

Reviewers: thanm, anna

Reviewed By: anna

Subscribers: anna, llvm-commits

Differential Revision: https://reviews.llvm.org/D53602

llvm-svn: 347050