[PowerPC] Do the Simple Early Return in block-placement pass to optimize the blocks

Summary:
In `block-placement` pass, it will create some patterns for unconditional we can do the simple early retrun.
But the `early-ret` pass is before `block-placement`, we don't want to run it again.
This patch is to do the simple early return to optimize the blocks at the last of `block-placement`. 

Below is an example
```
BB:                   | BB:
   XOR 3, 3, 4        |   XOR 3, 3, 4
   B TBB              |   B ChainBB
...                   | ...
ChainBB:              | ChainBB:
   B TBB              |   ADD 3, 3, 4
...                   |   BLR
TBB:                  |
   ADD 3, 3, 4        |
   BLR                |
```

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D63972

llvm-svn: 367080