[SystemZ] Move sign_extend optimization to PerformDAGCombine

The target was marking SIGN_EXTEND as Custom because it wanted to optimize
certain sign-extended shifts.  In all other respects the extension is Legal,
so it'd be better to do the optimization in PerformDAGCombine instead.

No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203234 91177308-0d34-0410-b5e6-96231b3b80d8