tree-optimization/106995 - if-conversion and vanishing loops

When we version loops for vectorization during if-conversion it
can happen that either loop vanishes because we run some VN and
CFG cleanup.  If the to-be vectorized part vanishes we already
redirect the versioning condition to the original loop.  The following
does the same in case the original loop vanishes as happened
for the testcase in the bug in the past (but no longer).

	PR tree-optimization/106995
	* tree-if-conv.cc (pass_if_conversion::execute): Also redirect the
	versioning condition to the original loop if this very loop
	vanished during CFG cleanup.