Fix multiple_p for two non-poly_ints

Fix multiple_p for two non-poly_ints

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.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@255860 138bc75d-0d04-0410-961f-82ee72b054a4