Use getEdgeProbability() instead of getEdgeWeight() in BFI and remove getEdgeWeight...

Use getEdgeProbability() instead of getEdgeWeight() in BFI and remove getEdgeWeight() interfaces from MBPI.

This patch removes all getEdgeWeight() interfaces from CodeGen directory. As
getEdgeProbability() is a little more expensive than getEdgeWeight(), I will
compose a patch soon in which BPI only stores probabilities instead of edge
weights so that getEdgeProbability() will have O(1) time.

Differential revision: http://reviews.llvm.org/D15489

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256039 91177308-0d34-0410-b5e6-96231b3b80d8