combine: Omit redundant AND in change_zero_ext.

combine: Omit redundant AND in change_zero_ext.

This is another micro-optimisation in change_zero_ext.  If an

  (and (lshiftrt ... (N)) (M))

generated by change_zero_ext is equivalent to just

  (lshiftrt ... (N))

(because the AND constant selects the N rightmost bits of the
result), strip off the AND.

gcc/ChangeLog:

2016-12-19  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	* combine.c (change_zero_ext): Skip generation of redundant AND.

From-SVN: r243792