[LoopUnswitch] Add block frequency analysis to recognize hot/cold regions

Summary: This patch adds block frequency analysis to LoopUnswitch pass to recognize hot/cold regions. For cold regions the pass only performs trivial unswitches since they do not increase code size, and for hot regions everything works as before. This helps to minimize code growth in cold regions and be more aggressive in hot regions. Currently the default cold regions are blocks with frequencies below 20% of function entry frequency, and it can be adjusted via -loop-unswitch-cold-block-frequency flag. The entire feature is controlled via -loop-unswitch-with-block-frequency flag and it is off by default.

Reviewers: broune, silvas, dnovillo, reames

Subscribers: davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D11605

llvm-svn: 248777