tree-optimization/117594 - fix live op vectorization for length masked case

tree-optimization/117594 - fix live op vectorization for length masked case

The code was passing factor == 0 to vect_get_loop_len which always
returns an unmodified length, even if the number of scalar elements
doesn't agree.  It also failed to insert the eventually generated
code.

	PR tree-optimization/117594
	* tree-vect-loop.cc (vectorizable_live_operation_1): Pass
	factor == 1 to vect_get_loop_len, insert generated stmts.

	* gcc.dg/vect/pr117594.c: New testcase.