Simplify the design of BranchProbabilityInfo by collapsing it into

a single class. Previously it was split between two classes, one
internal and one external. The concern seemed to center around exposing
the weights used, but those can remain confined to the implementation
file.

Having a single class to maintain the state and analyses in use will
also simplify several of the enhancements I want to make to our static
heuristics.

llvm-svn: 142783