[LoopUnroll] Handle certain PHIs in full unrolling properly

Summary:
When reconstructing the CFG of the loop after unrolling,
LoopUnroll could in some cases remove the phi operands of
loop-carried values instead of preserving them, resulting
in undef phi values after loop unrolling.

When doing this reconstruction, avoid removing incoming
phi values for phis in the successor blocks if the successor
is the block we are jumping to anyway.

Patch-by: ebevhan

Reviewers: fhahn, efriedma

Reviewed By: fhahn

Subscribers: bjope, lebedev.ri, zzheng, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66334

llvm-svn: 369886