1. Remove converting SDValue into int, rather operate via embedded APInt to

   avoid truncation.
2. Pass DAG instead of DCI to combineCCMask and SimplifyAssumingCCVals, and
   make these functions static instead of SystemZTargetLowering member function.
3. Treat SRL, SRA, SHL like other binary operators, and remove check for
   constant. This makes it even more generalized.
4. ADD, OR, XOR, AND, and SRA might clobber CC, added check hasOneUse().
5. Incorporated other code review feedback.

Author: anoopkg6