[BranchProbability] Remove the restriction that known and unknown probabilities canno...

[BranchProbability] Remove the restriction that known and unknown probabilities cannot coexist when being normalized.

The current BranchProbability::normalizeProbabilities() forbids known and
unknown probabilities to coexist in the list. This was once used to help
capture probability exceptions but has caused some reported build
failures (https://llvm.org/bugs/show_bug.cgi?id=25838).

This patch removes this restriction by evenly distributing the complement
of the sum of all known probabilities to unknown ones. We could still
treat this as an abnormal behavior, but it is better to emit warnings in
our future profile validator.

Differential revision: http://reviews.llvm.org/D15548

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255934 91177308-0d34-0410-b5e6-96231b3b80d8