LoopRotate: Add code to update branch weights

This adds code to the loop rotation transformation to ensure that the
computed block execution counts for the loop bodies are the same before
and after the transformation. This isn't always true in practice, but I
believe this is because of numeric inaccuracies in the BlockFrequency
computation.

The invariants this is modeled on and heuristic choice of 0-trip loop
amount is explained in a lenghty comment in the new
`updateBranchWeights()` function.

Differential Revision: https://reviews.llvm.org/D157462