fold-const.c (extract_muldiv): If not MULT_EXPR, check if either operand is divisible by C.

	* fold-const.c (extract_muldiv) [case PLUS_EXPR]: If not MULT_EXPR,
	check if either operand is divisible by C.
	(multiple_of_p): Handle LSHIFT_EXPR with small constant shift.
	If type is signed, consider negative numbers as well.

	* gcc.c-torture/execute/20010222-1.c: New test.

From-SVN: r39996