[MBP] Factor out the optimizations on branch conditions and unanalyzable branches. NFCI.

The benefits of this patch are

-- We call AnalyzeBranch() to optimize unanalyzable branches, but the result of
   AnalyzeBranch() is not used. Now the result is useful.

-- Before the layout of all the MBBs is set, the result of AnalyzeBranch() is
   not correct and needs to be fixed before using it to optimize the branch
   conditions. Now this optimization is called after the layout, the code used
   to fix the result of AnalyzeBranch() is not needed.

-- The branch condition of the last block is not optimized before. Now it is
   optimized.

Differential Revision: http://reviews.llvm.org/D20177

llvm-svn: 270623