[SelectionDAG] move constant or splat functions to common location

rL347502 moved the null sibling, so we should group all of these
together. I'm not sure why these aren't methods of the SDValue
class itself, but that's another patch if that's possible.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347523 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed