tree-optimization/108816 - vect versioning check split confusion

tree-optimization/108816 - vect versioning check split confusion

The split of the versioning condition assumes the definition is
in the condition block which is ensured by the versioning code.
But that only works when we actually have to insert any statements
for the versioning condition.  The following adjusts the guard
accordingly and asserts this condition.

	PR tree-optimization/108816
	* tree-vect-loop-manip.cc (vect_loop_versioning): Adjust
	versioning condition split prerequesite, assert required
	invariant.

	* gcc.dg/torture/pr108816.c: New testcase.

(cherry picked from commit 63471c5008819bbf6ec32a6f4d8701fe57b96fa9)