Rework vrp_int_const_binop interface

Rework vrp_int_const_binop interface

...to avoid a warning about uninitialised wide_ints.

2017-11-06  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vrp.c (vrp_int_const_binop): Return true on success and
	return the value by pointer.
	(extract_range_from_multiplicative_op_1): Update accordingly.
	Return as soon as an operation fails.

From-SVN: r254436