When trying to pass an argument on the stack, assume LLVM will do the right

thing for non-aggregate types.
 - Otherwise we unnecessarily pin values to the stack and currently end up
   triggering a backend bug in one case.

 - This loose cooperation with LLVM to implement the ABI is pretty ugly.

 - <rdar://problem/6918722> [irgen] clang miscompile of many pointer varargs on
   x86-64

llvm-svn: 72419