Increases full-unroll threshold.

Summary:
The default threshold for fully unroll is too conservative. This patch doubles the full-unroll threshold

This change will affect the following speccpu2006 benchmarks (performance numbers were collected from Intel Sandybridge):

Performance:

403	0.11%
433	0.51%
445	0.48%
447	3.50%
453	1.49%
464	0.75%

Code size:

403	0.56%
433	0.96%
445	2.16%
447	2.96%
453	0.94%
464	8.02%

The compiler time overhead is similar with code size.

Reviewers: davidxl, mkuper, mzolotukhin, hfinkel, chandlerc

Reviewed By: hfinkel, chandlerc

Subscribers: mehdi_amini, zzheng, efriedma, haicheng, hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D28368

llvm-svn: 295538