[GC] Lower vectors-of-pointers directly by default

This commit changes the default on our lowering of vectors-of-pointers from splitting in RS4GC to reporting them in the final stack map.  All of the changes to do so are already in place and tested.  Assuming no problems are unearthed in the next week, we will be deleting the old code entirely next Monday.

llvm-svn: 258111