[46/77] Make widest_int_mode_for_size return a scalar_int_mode

The comment for widest_int_mode said that it returns "the widest integer
mode no wider than SIZE", but it actually returns the widest integer
mode that is narrower than SIZE.  In practice SIZE is always greater
than 1, so it can always pick QImode in the worst case.  The VOIDmode
paths seem to be dead.

gcc/
2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* expr.c (widest_int_mode_for_size): Make the comment match the code.
	Return a scalar_int_mode and assert that the size is greater than
	one byte.
	(by_pieces_ninsns): Update accordingly and remove VOIDmode handling.
	(op_by_pieces_d::op_by_pieces_d): Likewise.
	(op_by_pieces_d::run): Likewise.
	(can_store_by_pieces): Likewise.

From-SVN: r251498