Factor the computation of input and output sets into a public interface

of the CodeExtractor utility. This allows speculatively computing input
and output sets to measure the likely size impact of the code
extraction.

These sets cannot be reused sadly -- we mutate the function prior to
forming the final sets used by the actual extraction.

The interface has been revamped slightly to make it easier to use
correctly by making the interface const and sinking the computation of
the number of exit blocks into the full extraction function and away
from the rest of this logic which just computed two output parameters.

llvm-svn: 156168