middle-end/97554 - avoid overflow in alloc size compute

middle-end/97554 - avoid overflow in alloc size compute

This avoids overflow in the allocation size computations in
sbitmap_vector_alloc when the result exceeds 2GB.

2020-10-26  Richard Biener  <rguenther@suse.de>

	* sbitmap.c (sbitmap_vector_alloc): Use size_t for byte
	quantities to avoid overflow.