Create a new ARM-specific DAG node, VDUP, to represent a splat from a

Create a new ARM-specific DAG node, VDUP, to represent a splat from a
scalar_to_vector.  Generate these VDUP nodes during legalization instead
of trying to recognize the pattern during selection.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78994 91177308-0d34-0410-b5e6-96231b3b80d8