tree-chrec: Use signed_type_for in convert_affine_scev

tree-chrec: Use signed_type_for in convert_affine_scev

On s390x-linux I've run into the gcc.dg/torture/bitint-27.c test ICEing in
build_nonstandard_integer_type called from convert_affine_scev (not sure
why it doesn't trigger on x86_64/aarch64).
The problem is clear, when ct is a BITINT_TYPE with some large
TYPE_PRECISION, build_nonstandard_integer_type won't really work on it.

The patch fixes it similarly what has been done for GCC 14 in various
other spots.

2025-05-20  Jakub Jelinek  <jakub@redhat.com>

	* tree-chrec.cc (convert_affine_scev): Use signed_type_for instead of
	build_nonstandard_integer_type.