Add trace accessor methods, implement primitive if-conversion heuristic.

Add trace accessor methods, implement primitive if-conversion heuristic.

Compare the critical paths of the two traces through an if-conversion
candidate. If the difference is larger than the branch brediction
penalty, reject the if-conversion. If would never pay.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161433 91177308-0d34-0410-b5e6-96231b3b80d8