[LoopOptimizer][Fix]PR32859, PR24738

The Loop vectorizer pass introduced undef value while it is fixing output of LCSSA form.
Here it is:

before: %e.0.ph = phi i32 [ 0, %for.inc.2.i ]
after: %e.0.ph = phi i32 [ 0, %for.inc.2.i ], [ undef, %middle.block ]

and after this change we have:

%e.0.ph = phi i32 [ 0, %for.inc.2.i ]
%e.0.ph = phi i32 [ 0, %for.inc.2.i ], [ 0, %middle.block ]

Committed on behalf of @dtemirbulatov

Differential Revision: https://reviews.llvm.org/D33055

llvm-svn: 302988