Fix PR12312

- Add a target-specific DAG optimization to recognize a pattern PTEST-able.
  Such a pattern is a OR'd tree with X86ISD::OR as the root node. When
  X86ISD::OR node has only its flag result being used as a boolean value and
  all its leaves are extracted from the same vector, it could be folded into an
  X86ISD::PTEST node.

llvm-svn: 162735