Factor the logic for testing whether a basic block is viable for code

Factor the logic for testing whether a basic block is viable for code
extraction into a public interface. Also clean it up and apply it more
consistently such that we check for landing pads *anywhere* in the
extracted code, not just in single-block extraction.

This will be used to guide decisions in passes that are planning to
eventually perform a round of code extraction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156114 91177308-0d34-0410-b5e6-96231b3b80d8