SDAG: Implement Select instead of SelectImpl in HexagonDAGToDAGISel

- Where we were returning a node before, call ReplaceNode instead.
- Where we had already replaced all uses and we returned a node, just
  remove the dead node instead.
- Where we would return null to fall back to another selector, rename
  the method to try* and return a bool for success.

Part of llvm.org/pr26808.

llvm-svn: 269358