Move the DEBUG uniqueness check outside of for-loop inside IGListAdapter
Move the DEBUG uniqueness check outside of for-loop inside IGListAdapter

Summary:
The original DEBUG was put inside a for loop, which caused a quadratic looping while creating bigger and bigger NSSet.

This is very inefficient and I verified that by profiling with 10000 objects in the array and it caused significant amount of CPU. The main thread is pretty much unresponsive.
Closes https://github.com/Instagram/IGListKit/pull/1084

Reviewed By: rnystrom

Differential Revision: D6903043

Pulled By: lorixx

fbshipit-source-id: 311e8a402eb8d5574fce0eabd626a674b6a5e8c5

Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit