alias: Perform offset arithmetics in poly_offset_int rather than poly_int64 [PR118819]

This PR is about ubsan error on the c - cx1 + cy1 evaluation in the first
hunk.

The following patch hopefully fixes that by doing the additions/subtractions
in poly_offset_int rather than poly_int64 and then converting back to poly_int64.
If it doesn't fit, -1 is returned (which means it is unknown if there is a conflict
or not).

2025-02-27  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/118819
	* alias.cc (memrefs_conflict_p): Perform arithmetics on c, xsize and
	ysize in poly_offset_int and return -1 if it is not representable in
	poly_int64.