middle-end: don't pass loop_vinfo to vect_set_loop_condition during prolog peeling

During the refactoring I had passed loop_vinfo on to vect_set_loop_condition
during prolog peeling.  This parameter is unused in most cases except for in
vect_set_loop_condition_partial_vectors where it's behaviour depends on whether
loop_vinfo is NULL or not.  Apparently this code expect it to be NULL and it
reads the structures from a different location.

This fixes the failing testcase which was not using the lens values determined
earlier in vectorizable_store because it was looking it up in the given
loop_vinfo instead.

gcc/ChangeLog:

	PR tree-optimization/111866
	* tree-vect-loop-manip.cc (vect_do_peeling): Pass null as vinfo to
	vect_set_loop_condition during prolog peeling.