Remove SCEVCache and FindConstantPointers from complete loop unrolling heuristic.

Summary:
Using some SCEV functionality helped to entirely remove SCEVCache class and FindConstantPointers SCEV visitor.
Also, this makes the code more universal - I'll take advandate of it in next patches where I start handling additional types of instructions.

Test Plan: Tests would be submitted in subsequent patches.

Reviewers: atrick, chandlerc

Reviewed By: atrick, chandlerc

Subscribers: atrick, llvm-commits

Differential Revision: http://reviews.llvm.org/D10205

llvm-svn: 239282