[LoopUnroll] Enable advanced unrolling analysis by default.

Summary:
This patch turns on LoopUnrollAnalyzer by default. To mitigate compile
time regressions, I chose very conservative thresholds for now. Later we
can make them more aggressive, but it might require being smarter in
which loops we're optimizing. E.g. currently the biggest issue is that
with more agressive thresholds we unroll many cold loops, which
increases compile time for no performance benefit (performance of those
loops is improved, but it doesn't matter since they are cold).

Test results for compile time(using 4 samples to reduce noise):
```
MultiSource/Benchmarks/VersaBench/ecbdes/ecbdes 5.19%
SingleSource/Benchmarks/Polybench/medley/reg_detect/reg_detect  4.19%
MultiSource/Benchmarks/FreeBench/fourinarow/fourinarow  3.39%
MultiSource/Applications/JM/lencod/lencod 1.47%
MultiSource/Benchmarks/Fhourstones-3_1/fhourstones3_1 -6.06%
```

I didn't see any performance changes in the testsuite, but it improves
some internal tests.

Reviewers: hfinkel, chandlerc

Subscribers: llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D20482

llvm-svn: 270478