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

[CodeGen] 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`.

Reviewed By: efriedma

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368509 91177308-0d34-0410-b5e6-96231b3b80d8