tree-optimization/116380 - bogus SSA update with loop distribution

tree-optimization/116380 - bogus SSA update with loop distribution

When updating LC PHIs after copying loops we have to handle defs
defined outside of the loop appropriately (by not setting them to
NULL ...).  This mimics how we handle this in the SSA updating
code of the vectorizer.

	PR tree-optimization/116380
	* tree-loop-distribution.cc (copy_loop_before): Handle
	out-of-loop defs appropriately.

	* gcc.dg/torture/pr116380.c: New testcase.