Remove the successor probabilities normalization in tail duplication pass.

Remove the successor probabilities normalization in tail duplication pass.

The normalization may cause assertion failures on SystemZ and some out-of-tree
tests. The root cause is that unknown probabilities are materialized into known
ones by calling getSuccProbability(), which is then used to add another
successor to the same MBB which results in mixed known and unknown
probabilities. But currently those mixed probabilities cannot be normalized.

I will compose another patch to fix the root issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255530 91177308-0d34-0410-b5e6-96231b3b80d8