Change DAG nodes for Neon VLD2/3/4 operations to return multiple results.

Change DAG nodes for Neon VLD2/3/4 operations to return multiple results.
Get rid of yesterday's code to fix the register usage during isel.
Select the new DAG nodes to machine instructions.  The new pre-alloc pass
to choose adjacent registers for these results is not done, so the
results of this will generally not assemble yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78136 91177308-0d34-0410-b5e6-96231b3b80d8