Assign correct edge weights to unwind destinations when lowering invoke statement.

When lowering invoke statement, all unwind destinations are directly added as successors of call site block, and the weight of those new edges are not assigned properly. Actually, default weight 16 are used for those edges. This patch calculates the proper edge weights for those edges when collecting all unwind destinations.

Differential revision: http://reviews.llvm.org/D13354

llvm-svn: 250119