[DAGCombiner] PCMP* sets its result to all ones or zeros so we can AND with the
shifted mask rather than masking and shifting separately.

The patch adds this transformation to the DAGCombiner:

  (shl (and (setcc:i8v16 ...) N01C) N1C) -> (and (setcc:i8v16 ...) N01C<<N1C)

<rdar://problem/16054492>

Patch by Adam Nemet <anemet@apple.com>