[PPC] Correctly adjust branch probability in PPCReduceCRLogicals

In PPCReduceCRLogicals after splitting the original MBB into 2, the 2 impacted branches still use original branch probability. This is unreasonable. Suppose we have following code, and the probability of each successor is 50%.

condc = conda || condb br condc, label %target, label %fallthrough

It can be transformed to following,

br conda, label %target, label %newbb newbb: br condb, label %target, label %fallthrough

Since each branch has a probability of 50% to each successor, the total probability to %fallthrough is 25% now, and the total probability to %target is 75%. This actually changed the original profiling data. A more reasonable probability can be set to 70% to the false side for each branch instruction, so the total probability to %fallthrough is close to 50%.

This patch assumes the branch target with two incoming edges have same edge frequency and computes new probability fore each target, and keep the total probability to original targets unchanged.

Differential Revision: https://reviews.llvm.org/D62430