Move CFG building code to a new lib/MC/MCAnalysis library.

The new library is 150KB on a Release+Asserts build, so it is quiet a bit of
code that regular users of MC don't need to link with now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212209 91177308-0d34-0410-b5e6-96231b3b80d8