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

gcc/ChangeLog

+2016-12-19  Dominik Vogt  <vogt@linux.vnet.ibm.com>
+
+	* combine.c (change_zero_ext): Skip generation of redundant AND.
+
 2016-12-19  Krister Walfridsson  <krister.walfridsson@gmail.com>
 
 	* config/netbsd.h (LINK_EH_SPEC): Define.

gcc/combine.c

@@ -11289,8 +11289,13 @@ change_zero_ext (rtx pat)
       else
 	continue;
 
-      wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
-      x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
+      if (!(GET_CODE (x) == LSHIFTRT
+	    && CONST_INT_P (XEXP (x, 1))
+	    && size + INTVAL (XEXP (x, 1)) == GET_MODE_PRECISION (mode)))
+	{
+	  wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
+	  x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
+	}
 
       SUBST (**iter, x);
       changed = true;