[NFC][PowerPC] Improve the for loop in Early Return

Summary:

In `PPCEarlyReturn.cpp`
```
183       for (MachineFunction::iterator I = MF.begin(); I != MF.end();) {
184         MachineBasicBlock &B = *I++;
185         if (processBlock(B))
186           Changed = true;
187       }
```
Above code can be improved to:
```
184       for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E;) {
185         MachineBasicBlock &B = *I++;
186         Changed |= processBlock(B);
187       }
```

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D63800

llvm-svn: 364496