Teach DAGCombiner how to canonicalize dags according to the rule

 (shl (zext (shr A, X)), X) => (zext (shl (shr A, X), X)).

The rule only triggers when there are no other uses of the
zext to avoid materializing more instructions.

This helps the DAGCombiner understand that the shl/shr
sequence can then be converted into an and instruction.

llvm-svn: 191393