tree-optimization/111917 - bougs IL after guard hoisting

The unswitching code to hoist guards inserts conditions in wrong
places.  The following fixes this, simplifying code.

	PR tree-optimization/111917
	* tree-ssa-loop-unswitch.cc (hoist_guard): Always insert
	new conditional after last stmt.

	* gcc.dg/torture/pr111917.c: New testcase.