[SystemZ] Remove AND mask of bottom 6 bits when result is used for shift/rotate

On SystemZ, shift and rotate instructions only use the bottom 6 bits of the shift/rotate amount.
Therefore, if the amount is ANDed with an immediate mask that has all of the bottom 6 bits set, we
can remove the AND operation entirely.

Differential Revision: http://reviews.llvm.org/D21854

llvm-svn: 274650