tree-optimization/102659 - avoid undefined overflow after if-conversion

tree-optimization/102659 - avoid undefined overflow after if-conversion

The following makes sure to rewrite arithmetic with undefined behavior
on overflow to a well-defined variant when moving them to be always
executed as part of doing if-conversion for loop vectorization.

2021-10-11  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102659
	* tree-if-conv.c (need_to_rewrite_undefined): New flag.
	(if_convertible_gimple_assign_stmt_p): Mark the loop for
	rewrite when stmts with undefined behavior on integer
	overflow appear.
	(combine_blocks): Predicate also when we need to rewrite stmts.
	(predicate_statements): Rewrite affected stmts to something
	with well-defined behavior on overflow.
	(tree_if_conversion): Initialize need_to_rewrite_undefined.

	* gcc.dg/torture/pr69760.c: Adjust the testcase.
	* gcc.target/i386/avx2-vect-mask-store-move1.c: Expect to move
	the conversions to unsigned as well.