fold-const: Fix A <= 0 ? A : -A folding [PR95810]

fold-const: Fix A <= 0 ? A : -A folding [PR95810]

We folded A <= 0 ? A : -A into -ABS (A), which is for signed integral types
incorrect - can invoke on INT_MIN UB twice, once on ABS and once on its
negation.

The following patch fixes it by instead folding it to (type)-ABSU (A).

2020-06-24  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/95810
	* fold-const.c (fold_cond_expr_with_comparison): Optimize
	A <= 0 ? A : -A into (type)-absu(A) rather than -abs(A).

	* gcc.dg/ubsan/pr95810.c: New test.