combine: Query can_change_dest_mode before changing dest mode

As reported in https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02388.html .

Changing the mode of a hard register can lead to problems, or at least
it can make worse code if the result will need reloads.


	* combine.c (change_zero_ext): Only change the mode of a hard register
	destination if can_change_dest_mode holds for that.

From-SVN: r242812

gcc/ChangeLog

+2016-11-23  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	* combine.c (change_zero_ext): Only change the mode of a hard register
+	destination if can_change_dest_mode holds for that.
+
 2016-11-23  Jeff Law  <law@redhat.com>
 
 	* varasm.c (assemble_name): Increase buffer size for name.

gcc/combine.c

@@ -11230,7 +11230,8 @@ change_zero_ext (rtx pat)
       else if (GET_CODE (x) == ZERO_EXTEND
 	       && SCALAR_INT_MODE_P (mode)
 	       && REG_P (XEXP (x, 0))
-	       && HARD_REGISTER_P (XEXP (x, 0)))
+	       && HARD_REGISTER_P (XEXP (x, 0))
+	       && can_change_dest_mode (XEXP (x, 0), 0, mode))
 	{
 	  size = GET_MODE_PRECISION (GET_MODE (XEXP (x, 0)));
 	  x = gen_rtx_REG (mode, REGNO (XEXP (x, 0)));