Update edge weights properly when merging blocks in if-conversion.

In if-conversion, there is a utility function MergeBlocks() that is used to merge blocks. However, when new edges are built in this function the edge weight is either not provided or not updated properly, leading to a modified CFG with incorrect edge weights. This patch corrects this issue.

Differential Revision: http://reviews.llvm.org/D12513

llvm-svn: 248030