PGO branch weight: update edge weights in SelectionDAGBuilder.

When converting from "or + br" to two branches, or converting from
"and + br" to two branches, we correctly update the edge weights of
the two branches.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200431 91177308-0d34-0410-b5e6-96231b3b80d8