tree.def (CEIL_DIV_EXPR, [...]): Tweak comments.

	* tree.def (CEIL_DIV_EXPR, FLOOR_DIV_EXPR, ROUND_DIV_EXPR): Tweak
	comments.
	(TRUNC_MOD_EXPR, CEIL_MOD_EXPR, FLOOR_MOD_EXPR, ROUND_MOD_EXPR):
	Add comments on sign of the result.
	* fold-const.c (tree_binary_nonnegative_warnv_p) <FLOOR_MOD_EXPR>:
	Recurse on operand #1 instead of operand #0.
	<CEIL_MOD_EXPR>: Do not recurse.
	<ROUND_MOD_EXPR>: Likewise.

From-SVN: r229146