Only set branch weight during sample pgo annotation when max_weight of the branch...

Only set branch weight during sample pgo annotation when max_weight of the branch is non-zero. Otherwise use default static profile to set branch probability.

Summary: It does not make sense to set equal weights for all unkown branches as we have static branch prediction available.

Reviewers: dnovillo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24732

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281912 91177308-0d34-0410-b5e6-96231b3b80d8