PR sanitizer/81281
	* match.pd ((T)(P + A) - (T)P -> (T) A): Split into separate
	simplify for plus with :c added, and pointer_plus without that.
	((T)P - (T)(P + A) -> -(T) A): Likewise.  If type is integral
	with undefined overflow and the conversion is not widening,
	perform negation in utype and only convert to type afterwards.
	((T)(P + A) - (T)(P + B) -> (T)A - (T)B): Split into separate
	simplify for plus with :c added, and pointer_plus without that.
	If type is integral with undefined overflow and the conversion is
	not widening, perform minus in utype and only convert to type
	afterwards.  Move the last pointer_diff_expr simplify into the
	two outermost ifs.

	* gcc.c-torture/execute/pr81281.c: New test.
	* gcc.dg/pr81281-1.c: New test.
	* gcc.dg/pr81281-2.c: New test.
	* g++.dg/ubsan/pr81281.C: New test.
	* g++.dg/ubsan/pr81281-aux.cc: New test.

From-SVN: r255449