PR target/88234
	* config/rs6000/rs6000.c (rs6000_gimple_fold_builtin): For
	vec_add and vec_sub builtins, perform PLUS_EXPR or MINUS_EXPR
	in unsigned_type_for instead of vector integral type where overflow
	doesn't wrap.

	* gcc.dg/ubsan/pr88234.c: New test.

From-SVN: r266619