Fix a stupid inversion.  This function is very rarely used and was
mostly to help split patches up, which is why it didn't get picked
up during initial testing.

2017-12-20  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* poly-int.h (multiple_p): Fix handling of two non-poly_ints.

gcc/testsuite/
	* gcc.dg/plugin/poly-int-tests.h (test_nonpoly_multiple_p): New
	function.
	(test_nonpoly_type): Call it.

From-SVN: r255860