fold-const: Fix up multiple_of_p [PR112733]

We ICE on the following testcase when wi::multiple_of_p is called on
widest_int 1 and -128 with UNSIGNED.  I still need to work on the
actual wide-int.cc issue, the latest patch attached to the PR regressed
bitint-{38,39}.c, so will need to debug that, but there is a clear bug
on the fold-const.cc side as well - widest_int is a signed representation
by definition, using UNSIGNED with it certainly doesn't match what was
intended, because -128 as the second operand effectively means unsigned
131072 bit 0xfffff............ffff80 integer, not the signed char -128
that appeared in the source.

In the INTEGER_CST case a few lines above this we already use
    case INTEGER_CST:
      if (TREE_CODE (bottom) != INTEGER_CST || integer_zerop (bottom))
        return false;
      return wi::multiple_of_p (wi::to_widest (top), wi::to_widest (bottom),
                                SIGNED);
so I think using SIGNED with widest_int is best there (compared to the
other choices in the PR).

2023-11-29  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/112733
	* fold-const.cc (multiple_of_p): Pass SIGNED rather than
	UNSIGNED for wi::multiple_of_p on widest_int arguments.

	* gcc.dg/pr112733.c: New test.