ivopts: fix wide_int_constant_multiple_p when VAL and DIV are 0. [PR114932]

wide_int_constant_multiple_p tries to check if for two tree expressions a and b
that there is a multiplier which makes a == b * c.

This code however seems to think that there's no c where a=0 and b=0 are equal
which is of course wrong.

This fixes it and also fixes the comment.

gcc/ChangeLog:

	PR tree-optimization/114932
	* tree-affine.cc (wide_int_constant_multiple_p): Support 0 and 0 being
	multiples.