re PR tree-optimization/84946 (UBSAN: in mem_valid_for_store_merging ../../gcc/gimple-ssa-store-merging.c:3951)

	PR tree-optimization/84946
	* gimple-ssa-store-merging.c (mem_valid_for_store_merging): Compute
	bitsize + bitsize in poly_uint64 rather than poly_int64.

From-SVN: r258665