Thread the determination of branch prediction hit rates back through the if-conversio...

Thread the determination of branch prediction hit rates back through the if-conversion heuristic APIs.  For now,
stick with a constant estimate of 90% (branch predictors are good!), but we might find that we want to provide
more nuanced estimates in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115364 91177308-0d34-0410-b5e6-96231b3b80d8