Revert "Move function to obtain branch weights into the BranchInst class. NFC."

This reverts commit r223784 and copies the 'ExtractBranchMetadata' to CodeGenPrepare.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223795 91177308-0d34-0410-b5e6-96231b3b80d8