[LoopUnroll] Properly update loop structure in case of successful peeling.

[LoopUnroll] Properly update loop structure in case of successful peeling.

When peeling kicks in, it updates the loop preheader.
Later, a successful full unroll of the loop needs to update a PHI
which i-th argument comes from the loop preheader, so it'd better look
at the correct block. Fixes PR33437.

Differential Revision:  https://reviews.llvm.org/D37153

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311922 91177308-0d34-0410-b5e6-96231b3b80d8