[LoopFusion] Update second loop guard non loop successor phis incoming

blocks.

Summary: The current LoopFusion forget to update the incoming block of
the phis in second loop guard non loop successor from second loop guard
block to first loop guard block. A test case is provided to better
understand the problem.
Reviewed By: jdoerfert
Subscribers: hiraditya, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D81421