Enhance BranchProbabilityInfo::calcUnreachableHeuristics for InvokeInst

When identifying blocks post-dominated by an unreachable-terminated block
in BranchProbabilityInfo, consider only the edge to the normal destination
block if the terminator is InvokeInst and let calcInvokeHeuristics() decide
edge weights for the InvokeInst.

llvm-svn: 256028