[Branch probability] Recompute branch weights of tail-merged basic blocks.
BranchFolderPass was not correctly setting the basic block branch weights when
tail-merging created or merged blocks. This patch recomutes the weights of
tail-merged blocks using the following formula:
branch_weight(merged block to successor j) =
sum(block_frequency(bb) * branch_probability(bb -> j))
bb is a block that is in the set of merged blocks.
<rdar://problem/
16256423>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215135
91177308-0d34-0410-b5e6-
96231b3b80d8