[LV] Fix insertion point for shuffle vectors in first order recurrence

Summary:
In first order recurrence vectorization, when the previous value is a phi node, we need to
set the insertion point to the first non-phi node.
We can have the previous value being a phi node, due to the generation of new
IVs as part of trunc optimization [1].

[1] https://reviews.llvm.org/rL294967

Reviewers: mssimpso, mkuper

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D32969

llvm-svn: 302532