tree-optimization/114551 - loop splitting and undefined overflow

tree-optimization/114551 - loop splitting and undefined overflow

When loop splitting hoists a guard computation it needs to make sure
that can be safely evaluated at this place when it was previously
only conditionally evaluated.  The following fixes this for the
case of undefined overflow.

	PR tree-optimization/114551
	* tree-ssa-loop-split.cc (split_loop): If the guard is
	only conditionally evaluated rewrite computations with
	possibly undefined overflow to unsigned arithmetic.

	* gcc.dg/torture/pr114551.c: New testcase.