combine: Query can_change_dest_mode before changing dest mode

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