llvm-cov: Added -b option for branch probabilities.

This option tells llvm-cov to print out branch probabilities when
a basic block contains multiple branches. It also prints out some
function summary info including the number of times the function enters,
the percent of time it returns, and how many blocks were executed.

Also updated tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197198 91177308-0d34-0410-b5e6-96231b3b80d8