[SystemZ] computeKnownBitsForTargetNode() / ComputeNumSignBitsForTargetNode()

Improve/implement these methods to improve DAG combining. This mainly
concerns intrinsics.

Some constant operands to SystemZISD nodes have been marked Opaque to avoid
transforming back and forth between generic and target nodes infinitely.

Review: Ulrich Weigand
llvm-svn: 327765