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[LoopUnrollAnalyzer] Don't treat gep-instructions with simplified offset as…

Description

[LoopUnrollAnalyzer] Don't treat gep-instructions with simplified offset as simplified.

Summary:
Currently we consider such instructions as simplified, which is incorrect,
because if their user isn't simplified, we can't actually simplify them too.
This biases our estimates of profitability: for instance the analyzer expects
much more gains from unrolling memcpy loops than there actually are.

Reviewers: hfinkel, chandlerc

Subscribers: mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D17365