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

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

llvm-svn: 281912