expand: Expand x / y * y as x - x % y if the latter is cheaper [PR96696]

expand: Expand x / y * y as x - x % y if the latter is cheaper [PR96696]

The following patch tests both x / y * y and x - x % y expansion for the
former GIMPLE code and chooses the cheaper of those sequences.

2021-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96696
	* expr.c (expand_expr_divmod): New function.
	(expand_expr_real_2) <case TRUNC_DIV_EXPR>: Use it for truncations and
	divisions.  Formatting fixes.
	<case MULT_EXPR>: Optimize x / y * y as x - x % y if the latter is
	cheaper.

	* gcc.target/i386/pr96696.c: New test.